System and method for frequency offsetting of information communicated in MIMO-based wireless networks

ABSTRACT

A communications system includes a multiple-input/multiple-output (MIMO) architecture for high capacity switched mesh networks. The MIMO architecture has a plurality of radio frequency chains. One of the plurality of radio frequency chains is configured to apply a first frequency offset to a base frequency of an output signal to generate a first transmitting frequency; and another of the plurality of radio frequency chains being configured to apply a second frequency offset to the base frequency to generate a second transmitting frequency. The system uses the carrier frequency offset to lock the clock of the master subsystem to the clock of the slave subsystem, thereby enabling bandwidth expansion to be employed on the MIMO data streams.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/390,062, filed Feb. 20, 2009, now issued as U.S. Pat. No. 8,254,865,which is a continuation-in-part application of U.S. patent applicationSer. No. 11/399,536, filed Apr. 7, 2006, now issued as U.S. Pat. No.7,881,690, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to communication systems. Morespecifically, the present invention relates to a system and method forfrequency offsetting of information communicated in multipleinput/multiple output-based communication systems.

2. Discussion of Related Art

In wireless communication systems, efficient data transmission may beachieved using a multiple input/multiple output system (“MIMO” or “MIMOsystem”). At its simplest, a MIMO system employs a single transmitter ora plurality of chained transmitters (“chain” or “chains”) associatedwith multiple physical transmitting antennas to send simultaneouslymultiple data streams (“signals”) through a radio channel. The multipledata streams are received by multiple receiving antennas associated witha single receiver or a plurality of chained receivers (“chain” or“chains”).

This system results in better spatial utilization of the radio channelbandwidth. In turn, higher throughput, improved link reliability, andimproved spectral efficiency are achieved. A MIMO channel includeschannel impulse responses or channel coefficients in the flat fadingcase between different pairs of transmitting and receiving antennas. Asis known in the art, a MIMO system may be modeled asy=Hx+n  (Equation 1)where x and y are the transmit and receive sign vectors, respectively, nis the channel noise vector, and H is channel matrix.

MIMO systems are most useful in indoor environments where walls,ceilings, and furniture provide a rich multi-path environment, such thatthe channel matrix allows for multiple independent and orthogonalimpulse responses or spatial signatures. In such an environment, theMIMO technology is able to transmit multiple parallel and independentdata streams relying on the orthogonal elements of the channel matrix.MIMO systems deployed in highly scattering environments produce highranked H matrices resulting in higher MIMO capacities even when lowcorrelated antennas are used.

MIMO systems which have been developed for 4G IEEE 802.16e WiMAXsystems, have been optimized with two central goals in mind: (1) tomaximize/optimize spectral efficiency; and (2) to dynamically achieveimprovements in coverage gain or reach by reducing spectral efficiency.

For cellular vendors, spectrum is a precious and limited resource whererevenue is defined largely as a function of system capacity andthroughput. Spectral efficiency is therefore of paramount importance forthese networks where revenue is measured as a functions of carriedbandwidth. A significant portion of a cellular provider's operatingexpenses are from the monthly leasing fees for each cell site.

Maintaining existing cell site coverage is also critical since ubiquityof service is a requirement for any 4G wireless network, and yet theincreased delivered channel bandwidth would have reduced link budgetsand therefore smaller cell sizes. Cellular providers rely on MIMOtechnology and the ability to tradeoff capacity for reach at the celledge to maintain the current cell coverage.

Cellular providers, which are by far the largest economic force drivingthe advancement of MIMO systems, have maintained the industry focus onspectral efficiency and dynamic reach tradeoff, as well as innovativeantenna systems at the base station (BS) and station set (SS) equipment.Those in the WiMAX industry are familiar with “Matrix A” for coveragegain—where a single data stream is transmitted in parallel over twoindependent transmitter-antenna-receiver paths using space time blockcodes (STBC) to encode the two streams such that they are orthogonal toeach other, thereby improving the signal-to-noise ratio (SNR) at thereceiver, resulting in increased cell radius. “Matrix B” was developedfor capacity increases which use the spatial multiplexing of MIMO totransmit independent data streams with throughput capacity limited onlyby the rank of the H matrix and the local noise floor characteristics.

MIMO systems which have been developed for IEEE 802.11n wideband localarea network (WLAN) systems have been optimized with the same twocentral goals driven by the cellular industry's 4G systems—maximizingspectral efficiency and capacity; and optimizing coverage. However, WLANvendors have overriding industry requirements of solution size, powerand cost, as these chipsets are now being embedded in every laptop PCsold as well as in all of the new cellular telephones and personaldigital assistants (PDAs). WLAN solution providers have made incrediblegains since the first IEEE 802.11b radios were introduced less than adecade ago. WLAN solutions have progressed in the areas of capacity andrange as the WiFi standard has evolved from the 11 Mbps systems based onIEEE 802.11b with an effective throughput of 6 Mbps to the 54 Mbps OFDMsystems of IEEE 802.11a and IEEE 802.11g with an effective throughput inthe range of 25 Mbps. The introduction of IEEE 802.11n with MIMO hasbeen demonstrated to show peak throughputs as high as 300 Mbps andeffective throughputs equivalent to 100 Mbps for most house holdapplications where MIMO technology is able to perform well.

The same WLAN solutions providers have focused their efforts on costreduction, by fully integrating chips and radio frequency transmittersto the point that a single chip is able to support all softwarefunctions as well as transmit and receive with a zero-IF (ZIF)architecture. Power reduction of these single chip solutions has allowedfor a limited mini-PCI power budget of approximately 3W, supporting a3×3 IEEE MIMO 802.11n protocol with relatively high powered transmittersin the range of 17 dBm per channel, as high as 20 dBm with the typical<3 dBi gain antennas used in laptops or for WLAN consumer access points.The same WLAN vendors have been less interested in spectral efficiencyand have allowed channel sizes to increase from 20 MHz bandwidths to 40MHz bandwidths.

While MIMO systems operate best in indoor or highly scatteringenvironments which produces high ranked H matrices resulting in higherMIMO capacities, cellular systems employing MIMO are deployed outdoorsand often in line-of-sight (LoS) or near LoS (NLoS) applications. Highgain antennas—between 10 dBi to 30 dBi—may be used for long distancepoint-to-point links. It is not as well known in the industry that radioscattering, also called multipath interference, is related directly tothe beamwidth of the antenna such that high gain narrow beam antennaswill see less multipath interference as do lower gain wide beamwidthantennas. This less obvious fact makes logical sense, as high gainantennas have a narrow antenna beamwidth and therefore a small aperturecapable of receiving strong radio signals. Signals received by such anarrow aperture will, in fact, have traveled similar distances resultingin minimal multipath interference. Another way to understand this factis by considering the reception of a high energy RF “impulse” generatedby a transmitter and received by a receiving antenna. The impulse willbounce off of many obstacles, arriving with at the receiving antenna asan impulse response. A receiving antenna with a narrow aperture pointeddirectly at the source of the impulse will reject any of the longerdelay echoes of the impulse, which tend to come from sources which arenot directly in-line with the transmitter. Thus, outdoor high gaindirectional point-to-point MIMO systems cannot rely on multipathdispersion or radio scattering as a means of increasing the rank of thespatial H matrix; however, other means including spatial separation andpolarization diversity are possible.

Cellular vendors have long relied on spatial separation to achieveindependence of the multipath reflections for antenna diversityreceivers in outdoor environments. Many papers have been writtenregarding spatial separation of receiving antennas. In general, when thereceiving antenna is mounted at a low height and is close to reflectingand scattering objects, then a very small separation in the range of onehalf of a wavelength or just a few inches is required to achieve channelmultipath independence. However, when the receiving antennas are mountedhigh on towers or rooftops, as is most often the case, then smallseparations have no significant reduction of the correlation of themultipath signatures, and larger separations, on the order of meters,must be used to gain independence of the radio channels. Most antennasystems mounted on rooftops, cell towers, and other elevated structuresseparate diversity receive antennas by 2 meters or more to achieve pathindependence to realize gains from antenna diversity. MIMO accesssystems can rely on the same antenna separation to improve overallthroughput.

Polarization diversity can also be used to achieve independence of theradio channel. Most IEEE 802.16e MIMO systems currently being deployeduse slant diversity in each of the antennas, and three or more antennasseparated by 2 meters each can achieve gains of beam steering as well asa high order channel matrix H. Unfortunately, wireless backhaul networkscannot afford to have multiple receiver antennas separated by two ormore meters because of the existing lease agreements for antennaattachment. These lease agreements typically limit an antenna to be lessthan 1 ft×1 ft×4 ft in total size, including the transceiver equipmentitself.

Moreover, cellular providers have further restricted equipmentmanufacturers of point-to-point radio equipment for backhaul purposes tobe less than 1 ft×1 ft×1 ft in total size, and this has become anindustry “norm” for such equipment. This restriction effectively limitsthe allowed antenna gain, but allows for antenna diversity to be used toachieve independence of the MIMO paths and allows for as high as a 2×2matrix.

Antenna polarization diversity works well for links which are LoS withno possibility of obstacles within the Fresnel zone of the radio. Insuch cases, the MIMO gains can be determined a priori so that thenetwork planner is able to accurately define how many radio links andtheir specified bandwidth that will be achieved using the 1 ft×1 ft×1 fttransceivers.

For the case of non-LoS or near LoS point-to-point links that experiencetime varying reflections, MIMO gains are less well characterized and mayonly be a fraction of the maximum possible throughput. As an example, a2×2 MIMO transmission formed using antenna polarization diversity willsee continuous polarization rotations if the signals pass through wetfoliage, such as trees, after a rainfall. The presence of a few trees inthe Fresnel zone typically results in a 10 dB reduction in transmittersignal strength, a condition such that even a light breeze can changethe propagation channel more quickly than the hardware algorithms areable to handle and update the channel matrix “H” to maintain fullthroughput. As a result, for these types of links, the MIMO gains aredifficult to quantify for network capacity planning.

Given the difficulties in quantifying the capacity for a 2×2 MIMOpoint-to-point radio link, the effort becomes even more challenging witha 3×3 or 4×4 MIMO solution. These higher MIMO solutions under idealconditions deliver significantly higher capacity than a non-MIMOsolution, yet their effectiveness is governed by site-specific issues ofLoS and near-LoS path characteristics. There are no documentedprocedures or guidelines which specify assured/minimum MIMO gains for agiven antenna separation; therefore, the installer and network plannerhas no accurate means to know before deploying the MIMO radios what thelinks capacity will be.

Finally, even under the best conditions of LoS and antenna isolation andseparation, interference in an unlicensed band is always an issue. Inmany environments, unlicensed band interference can be described as ageneral noise floor, driven by tens, hundreds, or thousands ofindividual and geographically dispersed sources, where typically just afew sources dominate.

The vast majority of interference sources tend to be in a fixedlocation—e.g., radiation from microwave ovens or DECT wireless phones,or even pinball machines. Some are mobile, such as Bluetooth devices orlaptops. In general, for outdoor point-to-point networks, the noisefloor tends to be static in nature, but with sudden changes when amobile source is introduced near to the point-to-point microwave link.These sources are not well handled by MIMO radio links which are channelspecific and are thus affected on all MIMO paths by a singleinterference source.

Thus, there is a need for an improved MIMO system that provides forgreater bandwidth and greater assured reliability. There is also a needfor a MIMO system that requires limited antennas to permit usability inlimited physical spaces.

In MIMO based technologies such as IEEE 802.11n Wi-Fi or IEEE 802.16eWiMAX, the transmitters have been designed to generate multiple outputdata streams using common crystal oscillators for the baseband andcommon local oscillator(s) (LOs) for the conversion to radio frequency(RF), and where the final RF signals are at the same frequencies. Thephase variations present in the baseband and LO circuits will be seenequally on all of the MIMO RF signals so that a MIMO receiver canrecover timing from any one of the MIMO RF signals and apply that timingto all of the other streams.

For example, a MIMO transmitter generates multiple MIMO RF signals at 5GHz using a crystal with a +10 parts per million (ppm) error. The RFsignals are transmitted over the air at 5 GHz+10 ppm=5,000,050,000 Hz.The MIMO receiver would receive the multiple RF signals using a crystalwith a −10 ppm error, so that the down conversion would be with a4,999,950,000 Hz signal. The resulting signal at baseband would have afrequency error of 100,000 Hz=100 kHz on all MIMO streams, which iseasily tracked and removed by the timing recovery function on any one ofthe recovered MIMO signals.

However, if the system shifts frequency of the MIMO streams to differentradio frequencies, a new problem occurs. When these different RF streamsare down converted, the resulting errors (as measured in Hz) will bedifferent for each MIMO stream generated from the various RF signals.For example, using the frequency shifter, a 2×2 MIMO transmitter maygenerate two MIMO RF signals at 5 GHz and 6 GHz using a crystal with a+10 ppm error. These RF signals will be transmitted over the air at 5GHz+10 ppm=5,000,050,000 Hz and 6 GHz+10 ppm=6,000,060,000 Hz. The MIMOreceiver will receive the two RF signals using a crystal with a −10 ppmerror, so that the down conversion will be with a 4,999,950,000 Hzsignal and a 5,999,940,000 Hz signal on the first and second MIMOstreams respectively. The resulting signals at baseband will have afrequency error of 100 kHz for the first MIMO stream and 120 kHz for thesecond MIMO stream. If the receiver derives its timing recovery from thefirst MIMO stream, then the second MIMO stream will have an error of 120kHz−100 kHz=20 kHz. This 20 kHz error, as seen on a 250 us packet, willappear as 5 complete rotations of the OFDM constellation, thus makingtiming recovery impossible for any of the modulation rates. It is notedthat the +10 ppm and −10 ppm frequency errors used above are shown onlyfor purposes of a simplified example calculation. Typical WLAN devicesuse crystals having frequency errors within the range of +/−20 ppm.Further, the above example assumes that the MIMO receiver derives itstiming recovery from a single stream. If the MIMO receiver derives itstiming on a per-stream basis, then the exemplary 20 kHz frequency errormay be inconsequential if the MIMO receiver can support relatively largefrequency variations.

Accordingly, there is a need to address the problem of different downconversion frequency errors for frequency-shifted RF streams in MIMOsystems.

SUMMARY OF THE INVENTION

These and other objects are met by the current invention. Therein, acommunications system includes a multiple-input/multiple-outputarchitecture comprising a plurality of radio frequency chains, whereinone of the plurality of radio frequency chains is configured to apply afrequency offset to a base frequency of an output signal to generate atransmitting frequency.

In one aspect, the invention provides a method of synchronizing areceiver with a transmitter in a communications system comprising amultiple-input/multiple-output (MIMO) architecture. The architecturecomprises a first and a second radio frequency chain. The communicationssystem is configured to transceive at least two signals having apredetermined frequency separation. The method comprises the steps of:a) locking a frequency of the receiver to an external timing reference;and b) locking a frequency of the transmitter to the external timingreference. Each of steps a) and b) is carried out independently of oneanother. The external timing reference may comprise a Global PositioningSystem (GPS) timing reference or an IEEE 1588 timing reference. The GPStiming reference or the IEEE 1588 timing reference may be configured tobe frequency locked to a variable crystal oscillator. The variablecrystal oscillator may include a 40-MHz variable crystal oscillator.Alternatively, the GPS timing reference or the IEEE timing reference maybe configured to lock a plurality of transceivers such that the methodis usable in any of a point-to-point application, a point-to-multipointapplication, and a multipoint-to-multipoint application. The IEEE 1588timing reference may be configured to be frequency locked to a GPStiming reference. In yet another alternative, the IEEE 1588 timingreference may be configured to be frequency locked to a BuildingIntegrated Timing Source (BITS) reference.

In another aspect, the invention provides a method of synchronizing areceiver with a transmitter in a communications system comprising amultiple-input/multiple-output (MIMO) architecture. The architecturecomprises a first and a second radio frequency chain. The receiver isconnected to a controller. The communications system is configured totransceive at least two signals having a predetermined frequencyseparation. The method comprises the step of locking a frequency of thereceiver to a frequency of the transmitter by configuring the controllerto perform the steps of: a) using a packet transmitted by thetransmitter and received by the receiver to generate a carrier frequencyoffset (CFO) estimate; b) adjusting the receiver reference frequencybased on the CFO estimate; and c) repeating steps a) and b) until thegenerated CFO estimate is substantially equal to zero. The controllermay comprise at least one of an open-loop controller; a closed-loopcontroller; a proportional controller; an integral controller; aderivative controller; and a Kalman filter.

In yet another aspect, the invention provides a method of synchronizinga receiver with a transmitter in a communications system comprising amultiple-input/multiple-output (MIMO) architecture. The architecturecomprises a first and a second radio frequency chain. The transmitter isconnected to a controller. The communications system is configured totransceive at least two signals having a predetermined frequencyseparation. The method comprises the step of locking a frequency of thetransmitter to a frequency of the receiver by configuring the controllerto perform the steps of: a) using a packet transmitted by the receiverand received by the transmitter to generate a carrier frequency offset(CFO) estimate; b) adjusting the transmitter reference frequency basedon the CFO estimate; and c) repeating steps a) and b) until thegenerated CFO estimate is substantially equal to zero. The controllermay comprise at least one of an open-loop controller; a closed-loopcontroller; a proportional controller; an integral controller; aderivative controller; and a Kalman filter.

In still another aspect, the invention provides a method ofsynchronizing a receiver with a transmitter in a communications systemcomprising a multiple-input/multiple-output (MIMO) architecture. Thearchitecture comprises a first and a second radio frequency chain. Thereceiver is connected to a controller. The communications system isconfigured to transceive at least two signals having a predeterminedfrequency separation. The method comprises the step of locking afrequency of the receiver to a frequency of the transmitter byconfiguring the controller to perform the steps of: a) using a packettransmitted by the transmitter and received by the receiver to determinean error associated with the transmitted packet; b) adjusting thereceiver reference frequency based on the determined error; and c)repeating steps a) and b) until the determined error is substantiallyequal to zero.

In yet another aspect, the invention provides a method of synchronizinga receiver with a transmitter in a communications system comprising amultiple-input/multiple-output (MIMO) architecture. The architecturecomprises a first and a second radio frequency chain. The receiver isconnected to a controller. The communications system is configured totransceive at least two signals having a predetermined frequencyseparation. The method comprises the step of locking a frequency of thereceiver to a frequency of the transmitter by configuring the controllerto perform the steps of: a) using a packet transmitted by thetransmitter and received by the receiver to determine an errorassociated with the transmitted packet; b) adjusting the receiverreference frequency based on the determined error; c) using aretransmission of the received packet to determine an updated errorassociated with the retransmitted packet; and d) repeating steps b) andc) until the determined updated error is substantially equal to zero.

In still another aspect, the invention provides a method ofsynchronizing a receiver with a transmitter in a communications systemcomprising a multiple-input/multiple-output (MIMO) architecture. Thearchitecture comprises a first and a second radio frequency chain. Thecommunications system is configured to transceive at least two signalshaving a predetermined frequency separation. The method comprises thesteps of: a) locking a frequency of the receiver to a first highprecision reference frequency; and b) locking a frequency of thetransmitter to a second high precision reference frequency. Each of thefirst and second high precision reference frequency employs a referencecrystal having a maximum frequency error of plus-or-minus 5 parts permillion.

In yet another aspect, the invention provides a method of synchronizinga first receiver with a first transmitter and a second receiver with asecond transmitter in a communications system comprising amultiple-input/multiple-output (MIMO) architecture. The architectureincludes a first and a second radio frequency chain. The communicationssystem is configured to transceive at least two signals having apredetermined frequency separation. The method comprises the steps of:a) using the predetermined frequency separation to determine anacceptable range of error vector magnitudes; b) using the determinedrange of error vector magnitudes to determine a corresponding range ofphase variations; c) using the determined range of phase variations todetermine a corresponding range of clock recovery errors; and d)applying a predetermined real time control algorithm to lock a carrierfrequency offset of a received signal to within the determined range ofclock recovery errors.

Each of the at least two signals may have a carrier frequency within therange of 4.80 GHz to 6.00 GHz. In this instance, the predeterminedfrequency separation may be less than 1.10 GHz and greater than 0.90GHz; alternatively, the predetermined frequency separation may be lessthan 50 MHz and greater than 30 MHz. In another alternative, each of theat least two signals may have a carrier frequency within the range of2.30 GHz to 3.90 GHz. In this instance, the predetermined frequencyseparation may also be less than 1.10 GHz and greater than 0.90 GHz, orthe predetermined frequency separation may be less than 50 MHz andgreater than 30 MHz. Each of the at least two signals may be modulatedusing a technique selected from the group consisting of 64 QuadratureAmplitude Modulation (64 QAM), 256 QAM, and 1024 QAM.

Step d) may further include applying a least mean squared erroralgorithm to lock the carrier frequency offset. Alternatively, step d)may further include applying a Kalman filter algorithm to lock thecarrier frequency offset. Step d) may also further include using one ofa voltage controlled oscillator, a global positioning system (GPS)timing source, an IEEE 1588 timing reference source, or anoven-controlled temperature-compensated crystal oscillator to apply thealgorithm.

In another aspect, a system for enabling bandwidth expansion onmultiple-input/multiple-output (MIMO) data streams used in high-capacityswitched mesh networks is provided. The system comprises a mastersubsystem and a slave subsystem, each of the master and slave subsystemsincluding a respective transmitter, a respective receiver, and arespective local oscillator. The system is configured to receive atleast a first signal having a first carrier frequency and a secondsignal having a second carrier frequency, the first and second signalshaving a predetermined frequency separation. The system is configured toalign the slave local oscillator to the master local oscillator byapplying a real time control algorithm, the real time control algorithmhaving parameters relating to the predetermined frequency separation anda determined acceptable range of error vector magnitudes andcorresponding ranges of phase variations and clock recovery errors.

Each of the first and second carrier frequencies may be within the rangeof 4.80 GHz to 6.00 GHz. In this instance, the predetermined frequencyseparation may be less than 1.10 GHz and greater than 0.90 GHz;alternatively, the carrier frequency for each of the first and secondsignals may be less than 50 MHz and greater than 30 MHz. In anotheralternative, each of the first and second carrier frequencies may bewithin the range of 2.30 GHz to 3.90 GHz. In this instance, thepredetermined frequency separation may be less than 1.10 GHz and greaterthan 0.90 GHz; alternatively, the carrier frequency for each of thefirst and second signals may be less than 50 MHz and greater than 30MHz. Each of the first and second signals may be modulated using atechnique selected from the group consisting of 64 Quadrature AmplitudeModulation (64 QAM), 256 QAM, and 1024 QAM.

The system may be further configured to align the slave local oscillatorto the master local oscillator by applying either of a least meansquared error algorithm or a Kalman filter algorithm to lock the carrierfrequency offset. The system may further include a voltage controlledoscillator that is configured to align the slave local oscillator to themaster local oscillator by applying the real time control algorithm.Alternatively, the system may further include one of a globalpositioning system (GPS) timing source, an IEEE 1588 timing referencesource, or an oven-controlled temperature-compensated crystaloscillator, any one of which may be configured to align the slave localoscillator to the master local oscillator by applying the real timecontrol algorithm.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a portion of a communication network forcommunicating information wirelessly in accordance with one or moreembodiments of the present invention.

FIG. 2 is a schematic view of the communication system of FIG. 1comprising a plurality of radio frequency chains that on a transmissionside apply an independent frequency offset to a base frequency.

FIGS. 3 a and 3 b are schematic views of a filter circuit in accordancewith one or more embodiments of the present invention.

FIG. 4 is a schematic view of a communication system in accordance withone or more further embodiments of the present invention wherein thecommunications system comprises a radio frequency chain that on atransmission side utilizes a base frequency and one or more radiofrequency chains that on a transmission side apply an independentfrequency offset to a base frequency.

FIG. 5 is a schematic view of a communication system in accordance withone or more further embodiments of the present invention wherein thecommunications system comprises a plurality of single down-conversion orup-conversion radio frequency chains that on a transmission side applyan independent frequency offset to a base frequency.

FIG. 6 is a schematic view of a communication system in accordance withone or more further embodiments of the present invention wherein thecommunications system comprises a radio frequency chain that on atransmission side utilizes a base frequency and one or more singledown-conversion or up-conversion radio frequency chains that on atransmission side apply an independent frequency offset to a basefrequency.

FIG. 7 is a schematic view of a communication system in accordance withone or more further embodiments of the present invention wherein thecommunications system comprises a plurality of linked radio frequencychains that on a transmission side apply the same frequency offset to abase frequency.

FIG. 8 is a schematic view of a communication system in accordance withone or more further embodiments of the present invention wherein thecommunications system comprises a plurality of radio frequency chainsthat on a transmission side apply an independent frequency offset to abase frequency and utilize a combiner to combine a transmitting signal.

FIG. 9 a is a schematic view of a communication system in accordancewith one or more further embodiments of the present invention whereinthe communications system comprises a plurality of radio frequencychains that are configured to create a virtual antenna on a receiverside.

FIG. 9 b is a schematic view of a further embodiment of a communicationssystem of FIG. 9 a.

FIG. 9 c is a schematic view of a further embodiment of a communicationssystem of FIG. 9 a.

FIG. 9 d is a schematic view of a communication system in accordancewith one or more further embodiments of the present invention.

FIG. 10 a is a schematic view of a ZIF circuit in accordance with one ormore embodiments of the present invention.

FIG. 10 b is a schematic view of the ZIF circuit of FIG. 10 a inaccordance with one or more embodiments of the present invention.

FIG. 10 c is a schematic view of a detail of an RF chain of FIG. 10 b inaccordance with one or more further embodiments of the presentinvention.

FIG. 10 d is a schematic view of a detail of an RF chain of FIG. 10 b inaccordance with one or more further embodiments of the presentinvention.

FIG. 10 e is a schematic view of the ZIF circuit of FIG. 10 a inaccordance with one or more embodiments of the present invention.

FIG. 10 f is a schematic view of a detail of an RF chain of FIG. 10 e inaccordance with one or more further embodiments of the presentinvention.

FIG. 10 g is a schematic view of a detail of an RF chain of FIG. 10 e inaccordance with one or more further embodiments of the presentinvention.

FIG. 10 h is a schematic view of a detail of an RF chain of FIG. 10 e inaccordance with one or more further embodiments of the presentinvention.

FIG. 11 is a perspective view of a portion of a communications networkin accordance with one or more embodiments of the present invention.

FIG. 12 is a perspective view of the portion of the communicationsnetwork of FIG. 11 wherein user communication devices are operative withthe network.

FIG. 13 is a graphical illustration of several signals beingcommunicated by a MIMO system in accordance with one or more furtherembodiments of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

At the digital signal processing (DSP) level, conventionalmultiple-input/multiple-output (MIMO) systems employ the ability toadjust for small difference in phase and frequency variations caused bya multipath environment. More specifically, MIMO systems are typicallyable to account for the dynamic nature of the multipath environment, inwhich reflective elements are moving relative to one another. Generally,such motions have velocities in the range of a walking pace, i.e., up to2 meters per second. Vehicular motion may include velocities in a rangeof up to approximately 150 meters per second. Accordingly, MIMOalgorithms which account for frequency shifts caused by vehicular motionmust allow for frequency shifts of up to 3000 Hz, assuming thatf=approximately 6 GHz (given that v=f*λ, so for v=c=3×10⁸ m/s, λ=0.05 m,and then for Δv=150 m/s, Δf=150/0.05=3000 Hz).

Accordingly, although conventional MIMO systems can perform a modestlevel of phase and frequency adjustments, the range of adjustment isgenerally only up to a few kilohertz to account for the effects of themobile multipath environment. Additionally, many conventional MIMOreceivers have a design margin, which allows the DSP algorithms toadjust for as much as +/−5 kHz of frequency or phase variations.

However, as described in an example above, the frequency shifter for a2×2 MIMO transmitter generating two MIMO RF signals at 5 GHz and 6 GHzusing a crystal with a +10 ppm error will result in an error of 20 kHzbetween the two MIMO streams. If the crystal error is +20 ppm, theresulting frequency error is 40 kHz. Such frequency adjustments are wellbeyond the expected frequency variation that typically results from amobile multipath environment. Accordingly, the present invention isintended to address this problem by defining a means to mitigate thesignificant frequency and phase variations that result when MIMO RFstreams are independently frequency shifted, thereby enhancingthroughput and ensuring a level of system performance having anacceptably low error rate.

FIG. 1 shows a schematic view of a portion of a communication networkfor communicating information wirelessly in accordance with one or moreembodiments of the present invention. FIG. 2 is a schematic view of thecommunication system of FIG. 1 comprising a plurality of radio frequencychains that on a transmission side apply an independent frequency offsetto a base frequency.

A communication network 20 includes one or more communications systems100, illustrated generally as systems 100 a and 100 b, which are inwireless communication with each other. However, the present inventionis not limited specifically to wireless communications but may includeany other method and means for communication now known or yet to bedeveloped.

Each system 100, e.g., system 100 a, 100 b, may be a portion of acommunication device, automated device, and/or the like and disposed ina receiver, transmitter, transceiver circuit or device and/or the like.For example, system 100, i.e., system 100 a may be integrated in acellular, i.e., mobile, telephone and system 100 b may be integrated ina base station. Accordingly, system 100, i.e., 100 a, 100 b, may each beable to send and receive signals, as will be taught further herein.

System 100 is preferably configured to be operative using multipleinput/multiple output architecture (“MIMO” or “MIMO system”) toefficiently transmit data to another like or compatible system andwithin network 20 or any other associated or suitable network.Communications may be achieved according to any suitable communicationprotocol now known or yet to be developed. Thus, network 20 and/orsystem 100 may communicate using frequencies and protocols for any IEEE802.11 protocol or standard, including but not limited to 802.11a,802.11b, 802.11g and/or 802.11n used as Wireless Local Area Networks(“WLAN”); 802.16d Worldwide Interoperability for Microwave Access(“WiMAX”), 802.16e WiMAX; 4G; 3rd Generation Partnership Project(“3GPP”), or 3rd Generation Partnership Project 2 (“3GPP2”) standardsbased radio, or any other system or protocol.

As simplified for clarity in FIG. 1, a first system 100, e.g., system100 a, transmits multiple data streams via one or more transmittingantennas in a channel 104 to one or more receiving antennas of asuitable receiving system, such as a second system 100, e.g., system 100b, within or associated network 20. Thus, for simplicity, certaindrawing figures depict only one system 100 to illustrate both thetransmitter-side and receiver-side of system 100.

However, channel 104 comprises a plurality of radio frequency signals102, i.e., data streams, which are transmitted and/or received from/byone system 100 to/from a suitable communications system in a channelmatrix H as defined in Equation 1. Channel 104 may comprises a bandwidthsuitable for 802.16d and/or 802.16e protocol. Thus, channel 104 may havea bandwidth of 1.25 MHz; 2.5 MHz; 5 MHz; 7.5 MHz; 10 MHz and/or 20 MHz.Channel 104 may comprises a bandwidth suitable for 802.11n protocol.Thus, channel 104 may have a bandwidth of 5 MHz, 10 MHz, 20 MHz, and/or40 MHz. However, the bandwidth of channel 104 is not limited to theforegoing, but may include any suitable bandwidth.

System 100 preferably includes a MIMO architecture, e.g., chip set, thatcomprises a baseband media access controller 106, a zero intermediatefrequency communication circuit (“ZIF circuit”) 108, and a plurality ofreceiving and/or transmitting radio frequency chains 110 operable withone or more receiving and/or transmitting antennas 118. Readilyavailable off-the-shelf components may be used in system 100 for reasonsof economy and the ability to customize solutions to specific users.

The MIMO architecture may be defined by the number of transmitter sideradio frequency chains and receiver side radio frequency chains thatoperably connect one system 100 to another suitable system, such as asecond system 100. Thus, MIMO system having N number of transmitters andM number of receivers is an N×M MIMO system.

Baseband media access controller 106 may be any suitable controller forcontrolling access to network 20 and includes at least a network-uniqueidentification. Baseband media access controller 106 is in communicationwith ZIF circuit 108 and may be integrated with it. However, preferablybaseband circuit 106 is configured to be standalone.

ZIF circuit 108 may be configured as is known in the art, but preferablycomprises a circuit that, as will be taught, comprises one or moreembodiments and/or is compatible with one or more embodiments of systemsand methods taught in U.S. Ser. No. 11/399,536, filed Apr. 7, 2006,which is hereby incorporated by reference in its entirety for allpurposes.

Either or both of the baseband media access controller 106 or ZIFcircuit 108 may be part of and/or associated with other devices, such asarrays of digital signal processor elements as are known in the art withregard to high performance MIMO chip sets that provide more processingpower than are associated with off-the-shelf MIMO chip sets.

In accordance with one or more embodiments of the present invention, acontroller for system 100 may source and terminate the data to becommunicated, and may be configured to provide a single integratedfunction that includes control of all functions of system 100.

ZIF circuit 108 is in communication via one or more physical layeroutputs and inputs 109 with the plurality of radio frequency chains 110such that ZIF circuit 108 may be used in industry standard 802.11napplications. While the exemplary embodiment of FIGS. 1 and 2 illustratethree chains 110, any suitable number of chains of at least two chainsmay be utilized.

Preferably, to operate under an 802.11n protocol, system 100 comprisesthree chains 110, while when utilized in a network operating an 802.16dor 802.16e protocol, system 100 comprises four chains 110. Each chain110, i.e., chains 110 a, 110 b, and 110 c, preferably comprises afiltering module 112; a transmitter circuit 114 for transmitting signals102 over channel 104; and/or a receiver circuit 116 for receivingsignals 102 over a channel 104, depending on whether system 100 isconfigured to respectively transmit only, receive only, or both; and aswitch 140 for switching between transmitting and receiving mode.Preferably, chains 110 are configured to have a transmission side andreceiving side such that the chain may be utilized both to receive andtransmit.

Although system 100 is illustrated as a Time Division Duplexing (“TDD”)system with respect to a preferred operating means in a networkoperating in accordance with an 802.11n protocol, one skilled in the artwill recognize that inventive system 100 may be readily configured as aFrequency Division Duplexing (“FDD”) system and utilized with respect toa network operating under WiMAX protocol. For example, one skilled inthe art will recognize that the addition of one or more duplexers willpermit system 100 to be operable as an FDD system.

On a transmitting side, each chain 110 is preferably configured toreceive a common output signal having a predetermined frequency from aphysical layer of the MIMO architecture, such as physical layer output109. Each chain 110 down-converts the signal and applies an independentadjustment to the frequency, i.e., applies a frequency offset togenerate a signal 102 for transmitting comprising a frequency thatincludes an offset from the frequency of the common output signal.Preferably, the transmitting frequency of each chain is different thanat least one other transmitting frequency from at least one other chain.

On a receiving side, a respective at least one chain 110 is configuredto receive a signal 102 comprising a frequency having a frequency offsetand to up-convert the signal to a frequency usable by a controller, suchas ZIF circuit 108, and then to pass the signal to a physical layer ofthe MIMO architecture, such as physical layer input 109. For example,the up-converted frequency may be the same as frequency of the commonoutput signal or it may be different. However, for clarity, it will beassumed that the frequency of the up-converted signal is the frequencyof the common output signal, i.e., the common base frequency.

Filtering module 112 may comprise any suitable filtering module, butpreferably comprises a filtering module 112 a comprising a doubleconversion filtering process. Each filtering module 112 a preferablycomprises a first mixer circuit 130 in communication with output signal200 of ZIF circuit 108 via physical layer output 109. Output signal 200comprises common base frequency f₀ wherein respective receivers andtransmitters of system 100 are operable.

In accordance with one or more embodiments of the present invention,output signal 200 may correspond to the frequency output of ZIF circuit.Thus, for each of the chains, respective output signal 200 is providedat the same frequency, base frequency, i.e., first frequency f₀.However, output signal 200 may also be or be associated with a basebandfrequency produced by a baseband circuit, and/or an intermediatefrequency produced by an intermediate frequency output circuit.

Preferably, base frequency f₀ may be any suitable frequency that may beused to transmit signals in network 20. Thus, if network 20 is a networkusing protocol 802.11n, first frequency f₀ may be in the 2.4 GHz band.The intermediate frequency f_(IF), which is lower than the firstfrequency f₀, may be any suitable frequency at which filtering may beperformed. For example, if the first frequency f₀=2.4 GHz, then theintermediate frequency f_(IF) may be f₀−810 MHz=1.59 GHz, although theintermediate frequency f_(IF) may be any appropriate frequency that issuitably lower than the first frequency f₀. Thus, advantageously, thebandwidth is effectively expanded and greater data delivery is assured.

The first mixer circuit of each chain preferably down-converts signal200 from the common base frequency f₀ to an intermediate frequencyf_(IF) to generate a second signal 202. Each intermediate frequencyf_(IF) may be different than any other intermediate frequency in thesame chain and/or system.

A first filter circuit 132 is in communication with the output of thefirst mixer circuit and filters the down-converted signal 202 to afiltered down-converted signal 204. While filter circuit 132 maycomprise any suitable filter circuit or device that is capable offiltering noise, distortion, and other spurious from a signal, such asdown-converted signal 202 at any suitable frequency, filter circuit 132may also comprise a SAW filter or other suitable filter, such as ahamming filter, brick wall filter, ceramic filter, and/or the like.Filter circuit 132 may also comprise a SAW filter switch bank 170 astaught further herein.

Filtering module 112 a preferably includes a second mixer circuit 134 incommunication with an output of first filter circuit 132. Second mixercircuit 134 preferably is configured to up-convert the filtereddown-converted transmission signal 204 to a third frequency f₁, i.e.,the transmission frequency, to generate a filtered transmission signal206. Filtered transmission signal 206 comprises the transmission signal200 with the noise, distortion and other spurious signals removed orsubstantially reduced.

First and second mixer circuits 130 and 134 provide a double-conversionby translating the transmission signal 200 at first frequency f₀ to anintermediate frequency f_(IF) for filtering, and then translating theresulting filtered signal at intermediate frequency f_(IF) to a higher,third transmitting frequency f₁ for transmission. In this manner, anadjustment, which is independent from adjustments by another chain, ismade to the base frequency, i.e., a frequency offset is applied, togenerate a signal for transmitting. The signal comprises a frequencythat includes an offset from the frequency of the common output signal.

Therein, the first frequency f₀ and the third frequency f₁ may be, butpreferably is not, substantially identical, although the third frequencyf₁ may be any suitable frequency that is higher than the intermediatefrequency f_(IF) and the same or different frequency than the firstfrequency f₀. The difference in frequency between frequency f₀ andfrequency f₁ comprises the frequency offset.

The frequencies of the first frequency f₀ and the third frequency f₁will depend on such factors as, for example, the nature and type oftransmission scheme and protocol used, the transmission characteristicsof ZIF communication circuit 108 or other like transmitter, receiver,transceiver or communication circuit/device used, and other likefactors.

Filtered transmission signal 206 may be transmitted using transmittercircuit 114 or any suitable transmitter or communication circuit ordevice. The output of second mixer circuit 134 is preferably incommunication with transmitter circuit 114.

Transmitter circuit 114 may be configured to transmit the filteredtransmission signal 206. Preferably, transmitter circuit 114 includes asuitable bandpass filter 136 that receives and appropriately filters thefiltered transmission signal 206.

For example, for WiFi signals, the bandpass filter 136 may be used tofilter or otherwise limit the frequency width of filtered transmissionsignal 206 to the WiFi frequency band so as not to interfere with othersignals. The resulting bandpass-filtered signal is appropriatelyamplified or otherwise raised in power level by a power amplifier 138 orother suitable power amplifier in communication with the bandpass filter136.

Preferably, since the filtered transmission signal 206 is cleaner, by,for example, having a clean spectrum with little or no noise ordistortion, as a result of passing through the filtering module 112, thepower level of the signal may be raised to greater levels to increasethe transmission power without the concomitant increase in noise andother spurious signals.

The amplified signal 206 from power amplifier 138 can be passed to atransmitter/receiver diversity switch 140 for transmission via physicalantenna 118 using a suitable wireless transmission protocol, althoughthe amplified signal may be alternatively transmitted via an appropriatewired connection using a suitable wired protocol or standard.Transmitter circuit 114 and accompanying transmission components caninclude additional and/or alternative elements necessary for wireless orwired signal transmission, depending on, for example, the type ofsignals being transmitted, the communication medium and protocol, andother like factors.

Transmitter/receiver diversity switch 140 may instead comprise anoperative connection to separate receiving and transmitting antennas.

System 100 may be configured to receive wireless signals 102 via areceiving antenna, which can be the same or different antenna thanantenna 118. The signals received via the receiving antenna are passedvia the transmitter/receiver diversity switch 140 to a receiver circuit116.

Receiver circuit 116 is configured to receive signals for the system 100and may comprise a suitable bandpass filter 144, which receives andappropriately filters the received signals. For example, for WiFisignals, bandpass filter 144 may be used to filter or otherwise limitthe frequency width of the received signals to the WiFi frequency bandto remove out-of-band noise or other interfering signals.

The resulting bandpass-filtered signal is appropriately amplified by asuitable low-noise amplifier 144 in communication with bandpass filter142. Receiver circuit 116 and accompanying receiver components mayinclude additional and/or alternative elements necessary for wireless orwired signal reception, depending on, for example, the type of signalsbeing received, the communication medium and protocol, and other likefactors. The output of receiver circuit 116 is a received signal 208.Since signal 208, e.g., signal 102, is received from a like system, thefrequency of signal 208 preferably is the same as that of thetransmitted signal, e.g., the frequency of signal 208 comprisestransmitted frequency f₁.

Filtering module 112 a includes a third mixer circuit 146 which has aninput in communication with an output of the receiver circuit 116.Preferably, third mixer circuit 146 is configured to receive receivedsignal 208 at frequency f₁. Third mixer circuit 146 may be configured todown-convert the received signal 208 at frequency f₁ to an intermediatefrequency f_(IF) to generate a down-converted received signal 210.

Filtering module 112 a preferably includes a second filter circuit 148in communication with an output of third mixer circuit 146. Secondfilter circuit 148 is configured to filter the down-converted receivedsignal 210 to generate a filtered down-converted received signal 212 atthe intermediate frequency f_(IF), which may be a different intermediatefrequency than any other intermediate frequency in the same or differentchain or in system 100.

Second filter circuit 148 may comprise any suitable type of filtercircuit or device that is capable of filtering noise, distortion andother spurious signals from the down-converted received signal 210 atthe intermediate frequency f_(IF). Second filter circuit 148 may beconfigured substantially similar to first filter circuit 132.

Filtering module 112 a preferably includes a fourth mixer circuit 150 incommunication with an output of the second filter circuit 148. Fourthmixer circuit 150 is preferably configured to up-convert the filtereddown-converted received signal 212 to the base frequency f₀ to generatea filtered received signal 214. ZIP circuit 108 or other liketransmitter, receiver, transceiver or communication circuit/device is incommunication with an output of the fourth mixer circuit 150 viaphysical layer input 109.

Filtered received signal 214 comprises received signal 208 with thenoise, distortion and other spurious signals removed or substantiallyreduced. The third and fourth mixer circuits 146 and 150 providesdouble-conversion of the received signal 208 at transmitted frequency f₁to a lower, the intermediate frequency f_(IF) for filtering, and thentranslating the resulting filtered signal to a higher, base frequency f₀for reception by the ZIF circuit 108. In this manner, the frequencyoffset is reversed to generate a signal for use by a controller thatcomprises a frequency of the common output signal.

Filtering module 112 includes one or more local oscillator circuits 152in communication with the first, second, third and fourth mixer circuits130, 134, 146, and 150 to control the mixing frequencies of theplurality of mixer circuits.

However, local oscillator circuit 152 may use any suitable frequencycontrol signal or the like to control the mixing frequencies of each orany combination of the first, second, third and fourth mixing circuits130, 134, 146 and 150. Oscillator circuit 152 may comprise any suitabletype of RF oscillator circuit or the like, including a suitable PhaseLocked Loop (“PLL”) oscillator circuit or the like. Therein, all localoscillators are associated with a common frequency controller 111, i.e.clock, for controlling the respective mixing frequencies.

Chains 110 b and 110 c are configured similarly by varying theoscillation to produce transmitting frequency f₂ and f₃. In this manner,an adjustment, which is independent from adjustments by another chain,is made to the base frequency, i.e., a frequency offset is applied, togenerate a signal for transmitting having respective frequencies f₂ andf₃. Therein, the difference in frequency between frequency f₀ andfrequency f₂ or f₃ comprises the frequency offset. Similarly, chains 110b and 110 c are configured to receive frequencies f₂ and f₃ and reversethe frequency offset to generate a signal for use by a controller thatcomprises a base frequency f₀ of the common output signal.

Modifications and variations to filtering module 112, transmissionmodule 114, and/or receiving module 116 may be made by one skilled inthe art for increasing gain, achieving particular filtering, and/or anyother suitable purpose and such are contemplated in the presentinvention.

In accordance with one embodiment of the present invention, oneoscillator circuit 152, but not the other oscillator circuits, i.e., themaster oscillator circuit may comprise or be associated with frequencycontroller 111, i.e. clock, for controlling the respective mixingfrequencies. Frequency controller 111 may be disposed in any one of theoscillator circuits 152, but not the others, or in addition thereto maybe associated with ZIF circuit 108.

Each of the local oscillators may be configured to comprise differentoscillation frequency in cooperation with each of the respective chainsin system 100. In the exemplary embodiment of FIGS. 1 and 2, while onechain 110 produces a transmitting frequency f₁, a second chain 110 maygenerate a third frequency f₂ and chain 110 may generate a thirdfrequency f₃. Therein, each frequency f₁, f₂, and f₃ is offset from thebase frequency f₀ and each is different from the other.

Thus, for the exemplary embodiment of FIGS. 1 and 2, a 3×3 MIMO systemcomprises RF chains for individually adjusting the frequency of signals102 in a channel 104. The matrix for channel 104 is shown in Equation 2or more explicitly in Equation 3, wherein a superscript indicates thefrequency that has been offset.

$\begin{matrix}{H = \begin{bmatrix}h_{11} & 0 & 0 \\0 & h_{22} & 0 \\0 & 0 & h_{33}\end{bmatrix}} & \left( {{Equation}\mspace{14mu} 2} \right) \\{H = \begin{bmatrix}h_{11}^{f_{1}} & 0 & 0 \\0 & h_{22}^{f_{2}} & 0 \\0 & 0 & h_{33}^{f_{3}}\end{bmatrix}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$Therein, the matrix coefficients are expressed as h_(TR), where T is thetransmitting module on a respective chain, and R is the receiving moduleon a respective chain and indicated by numerals. It should beappreciated that if the number of chains that are present=n, the matrixmay be suitably adjusted.

Thus, h₂₂ comprises a transmission from one RF chain 110 of a system 100and received by a second RF chain 110 on a second system 100.Preferably, frequencies are selected such that the cross-products of onetransmitting chain to another receiving chain having a different offsetfrequency are almost zero due to frequency independence. For example,frequency offsets may range up to 60 Hz.

Preferably, frequencies chosen are adjacent channels, second adjacentchannels or similarly situated other channels. Many of the modulationtechniques used for MIMO systems comprise high levels of out-of-bandemissions that fall in the adjacent and next adjacent channel. Theemissions create high levels of co-channel interference, as for examplein the exemplary embodiment of FIG. 2, from frequencies f₀ to f₁, and f₁to f₀, use of these channels preferably includes filtering module 112.

For channels which are not adjacent or next adjacent, additionalfiltering is not required. Thus, signal 206 may be generated withoutfiltering to a new frequency by using a down-conversion or up-conversionin each chain. Thus, unlike the “integrated wireless transceiver”disclosed in U.S. Ser. No. 11/158,728, which was published as U.S.Patent Publication 2006/0292996 on Dec. 28, 2006, and which is herebyincorporated by reference for all purposes, a greater flexibility isoffered.

Alternatively, frequency offsets may also be produced using a basebandinput with a double heterodyne architecture such that the resultingfrequencies are different. As one skilled in the art will recognize, astandard high frequency radio design, which commonly in high power highperformance radio designs, and when developed over several chains offersa greater flexibility.

The frequencies may be in the same or different bands, and these bandsmay be licensed or unlicensed. In the unlicensed industrial, scientificand medical (“ISM”) radio bands, system 100 may comprise additionalcontrols for handling functionality such as dynamic frequency selection(“DFS”), with radar detection. For example, each receiver side of thefiltering module 112 may comprise a means to detect radar pulses to meetFCC or international rules for DFS. Preferably, the controls areconfigured to detect radar pulses on the unlicensed band frequencies anddynamically change channels if required. In contrast, a standard MIMOsystem that does not incorporate a frequency offset comprises only asingle radar detector since all MIMO operations are performed on acommon frequency.

In accordance with one or more embodiments of the present invention,further flexibility may be provided for system 100 by selectablyoperating local oscillator circuit 152 of chain as a common oscillatorcircuit is operatively connected to one or more of the other localoscillator circuits. Using a switch 149, the local oscillator circuitmay be bypassed in favor of a common oscillator circuit such that thesame frequency is generated. Advantageously, when necessary, system 100may be switched from a MIMO system to a standard system.

In accordance with one or more embodiments of the present invention,antenna 118 may comprise two separate antennas, a first antennacomprising inputs for vertical and horizontal polarization and secondcomprising a single or dual input. Therein, one frequency, frequency f₁,is connected to a first antenna, a second and third frequencies, e.g.,frequency f₂ and frequency f₃, are connected to a second antenna andwherein the polarizations of may be aligned or reversed to thepolarizations of at least one of the receiving chains and wherein athird frequency, e.g., frequency f₃, is aligned in polarization to atleast another of the receiving chains.

Antenna 118 may comprise two separate antennas with separate inputs forvertical and horizontal polarization, two separate antennas, each withinputs for circular polarization, and used in the fashion describedabove, two separate antennas, each with inputs for a common polarizationto allow beam steering of one frequency, frequency f₁, and non-beamsteering of a second frequency, frequency f₂, on the second antenna.Antenna 118 may also comprise one common antenna, with three inputs anda common polarization to allow beam steering of one frequency, but notthe other frequencies.

Advantageously, system 100 configured as a 3×3 MIMO system achieves abandwidth expansion by a factor of two to achieve an assured bandwidthfor this link which is roughly equivalent to that of two 2×2 MIMO systemwith a 1×1 single-input/single-output system.

FIGS. 3 a and 3 b are schematic views of a filter circuit in accordancewith one or more embodiments of the present invention. Filter circuit132 and/or filter circuit 148 may comprise SAW filter switch bank 170 inorder to improve the link budget of system 100 due to frequency offsetthat is applied. A switch bank 170 a may comprise a first SAW filter 172a and a second SAW filter 172 b placed in parallel to each other. Aplurality of switches 175 permit the selection of one or the otherfilter 172 and by routing signals, respectively, from or to a commoninput or output. Similarly, a switch bank 170 b may comprise a first SAWfilter 173 a and a second SAW filter 173 b connected in parallel to eachother and a third and fourth SAW filter 174 a and 174 b connected inparallel each other, respectively. A plurality of switches 175 permitthe selection of one or the other filter 173 or 174 by routing signals,respectively, from or to a common input or output.

For example, a channel 104 may be a standard MIMO channel of 20 MHz andcomprises a typical thermal noise floor of −174 dBm/Hz or −101 dBm.Using two MIMO streams with spatial diversity, the link budgets, e.g.the total of all of gains and losses from the transmitter to thereceiver, will be the same, assuming that the MIMO streams do not selfinterfere. The noise bandwidth will remain at 20 MHz or −101 dBm.

Using a single 40 MHz channel, the throughput will be equivalent to adual MIMO throughput for a 20 MHz channel, however, the link budget willsuffer by 3 dB as the 40 MHz wide noise floor will be at −98 dBm. Usingtwo independent 20 MHz channels, with a SAW filter circuit, the noisefloor per MIMO stream will be −101 dBm, ensuring that the link budgetsremain equivalent to a single 20 MHz channel, but using 40 MHz ofspectrum.

FIG. 4 is a schematic view of a communication system in accordance withone or more further embodiments of the present invention, wherein thecommunications system comprises a radio frequency chain that, on atransmission side, utilizes a base frequency, and one or more radiofrequency chains that, on a transmission side, apply an independentfrequency offset to a base frequency. System 100 c is preferablyconfigured to be operative using MIMO architecture to efficientlytransmit data between another system 100 c and/or other compatibleand/or suitably configured system. Thus, system 100 c may be operativewith other systems 100, such as systems 100 a and/or 100 b, taughtherein and comprises essentially like architecture to these system.Thus, the teachings of system 100, i.e., 100 a and/or 100 b, arerepeated here. However, system 100 c varies in certain aspects.

Advantageously, system 100 c provides a cost-effective solution bysimplifying the architecture and reducing the number of components.System 100 c permits the independent adjustment of the frequency of oneor more chains while one chain's frequency is linked to the ZIF circuit.In a network operating an 802.11 protocol, wherein three chains areused, at least two chains are independently adjustable to obtain adesired offset frequency while one chain's frequency is substantiallyidentical to the frequency of an output signal of ZIF circuit 108.Accordingly, rather than a having a chain 110 a that includes afiltering module 112, system 100 c includes a chain 110 d comprisingtransmitting module 114 and a receiving module 116 that in directcommunication with ZIF circuit 108, and a switch 140.

As disclosed, ZIF circuit 108 produces common output signal 200 at afrequency sufficient for transmission. Thus, first frequency f₀ may beequal to third frequency f₁ and may be transmitted as signal 102 afterappropriate amplification. Similarly, when received, signal 102 atfrequency f₁ is cleaned of spurious emissions by the receiving module116 and passed as signal 214 to the physical layer input 109 of ZIFcircuit 108. Accordingly, for the exemplary embodiment illustrated inFIG. 4, the exemplary MIMO system is a 3×3 system wherein the matrix ofchannel 104 is identical to Equations 2 and 3. To prevent an unintendedsignal delay between chain 110 d and chains 110 b and 110 c. i.e., thefiltered chains, ZIF circuit 108 preferably outputs signal 200 to chain100 d by an appropriate amount so that signal 102 is commonly timed.

In accordance with one embodiment of the present invention, system 100 cmay be configured so that one of the local oscillator circuits 152 is amaster oscillator circuit that permits both chains 110 b and 110 c tooutput the same frequency when a suitable switch 149 places the masteroscillator in operative control of the other chain's filtering module.In accordance with one embodiment of the present invention, chain 110 dmay be eliminated and ZIF circuit 108 is in direct communication withswitch 140.

FIG. 5 is a schematic view of a communication system in accordance withone or more further embodiments of the present invention, wherein thecommunications system comprises a plurality of single down-conversion orup-conversion radio frequency chains that, on a transmission side, applyan independent frequency offset to a base frequency. System 100 d ispreferably configured to be operative using MIMO architecture toefficiently transmit data between another system 100 d and/or othercompatible and/or suitably configured system. Thus, system 100 d may beoperative with other systems 100, such as systems 100 a and/or 100 b,and comprises essentially like architecture. Thus, the teachings ofsystem 100, i.e., 100 a and/or 100 b, are repeated here. However, system100 d varies in certain aspects.

Advantageously, system 100 d provides a cost-effective solution of asimplified architecture that reduces the number of components. System100 d permits the independent adjustment of the transmitting frequencyof one or more RF chains 110 with a single down-conversion orup-conversion. In a network operating an 802.11 protocol, wherein threechains are used, each chain may be independently adjustable to obtain adesired channel matrix.

System 100 d comprises a plurality of chains 110. Illustrated in FIG. 5are three chains 110 e, 110 f, and 110 g used in a network 20 operativewith an 802.11 protocol. However, any suitable number of chains may beused. Each of chains 110 e-110 g is configured substantially similar tochains 110 a-110 c. However, rather than comprising a respectivefiltering module 112 comprising a double conversion, i.e., filteringmodule 112 a, one or more chains 110 e-110 f comprise a respectivefiltering module 112, i.e., filtering module 112 e. By way of example,filtering module 112 e comprises a transmission-side mixer circuit 130,i.e., mixer circuit 130 e that down-converts an output signal 200 fromZIF circuit 108.

As taught herein, output signal 200 is preferably provided at a commonfrequency, i.e., first frequency f₀. Mixer circuit 130 e down-convertsfirst frequency f₀ to a suitable transmission frequency f₁ and passesthe signal 206 to a suitable transmitter circuit 114 for transmittingsignals 102 at a frequency f₁ via antenna 118 to an operativelycompatible system 100 for receiving.

Respective filtering module 112 e further comprises a receiver-sidemixer circuit 150, i.e., mixer circuit 150 e that up-converts a signal208. As discussed herein, signal 102 at a frequency f₁ is received viaantenna 118 from an operatively compatible system 100 and passed toreceiver circuit 116. Receiver circuit 116 cleans signal 102 and passesa cleaned signal 208 to mixer circuit 150 e for up-conversion to firstfrequency f₀. In turn, the mixer circuit passes signal 214 to ZIFcircuit 108. The filtering module further comprises a local oscillatorcircuit 152 e provides a suitable frequency control signal to localmixer circuits 130 e and 150 e. Filtering modules 110 f and 110 g arepreferably similarly configured to output a signal 102 at respectivefrequencies f₂ and f₃ and to receive the same frequencies. Accordingly,channel 104 is identical to Equations 2 and 3 for the exemplaryembodiment illustrated in FIG. 5. Accordingly, for the exemplaryembodiment illustrated in FIG. 5, the exemplary MIMO system is a 3×3system wherein the matrix of channel 104 is identical to Equations 2 and3.

In accordance with one embodiment of the present invention, system 100 dmay be configured so that one of the local oscillator circuits 152 e isa master oscillator circuit that permits one or more chains 110 f and110 g to output the same frequency when a suitable switch, such as aswitch 149, places the master oscillator in operative control of theother chain's filtering module. One skilled in the art will recognizethat other means for frequency offsetting and or frequency offsettingcircuits may also be employed and such are contemplated in the scope ofthe present invention.

FIG. 6 is a schematic view of a communication system in accordance withone or more further embodiments of the present invention, wherein thecommunications system comprises a radio frequency chain that, on atransmission side utilizes a base frequency, and one or more singledown-conversion or up-conversion radio frequency chains that, on atransmission side, apply an independent frequency offset to a basefrequency. System 100 e is preferably configured to be operative usingMIMO architecture to efficiently transmit data between another system100 e and/or other compatible and/or suitably configured system. Thus,system 100 e may be operative with other systems 100, such as systems100 a, 100 b, 100 c, and/or 100 d, and comprises essentially likearchitecture. Thus, the teachings of system 100, i.e., 100 a, 100 b, 100c, and/or 100 d, are repeated here. However, system 100 e varies incertain aspects.

Advantageously, system 100 e provides a cost-effective solution of asimplified architecture that reduces the number of components. System100 d permits the independent adjustments of the frequency of one ormore RF chains 110 with a single down-conversion or up-conversion. In anetwork operating an 802.11 protocol, wherein three chains are used, atleast two chains are independently adjustable to obtain a desired offsetfrequency while one chain's frequency is substantially identical to thefrequency of an output signal of ZIF circuit 108.

System 100 e comprises a plurality of chains 110. Illustrated in FIG. 6are three chains 110 h, 110 i, and 110 j that are used in a network 20operative with an 802.11 protocol. However, any suitable number ofchains may be used. System 100 e includes a chain 110 h that isconfigured substantially identical to chain 110 d, wherein the chaincomprises transmitting module 114 and a receiving module 116 that indirect communication with ZIF circuit 108, and a switch 140 as taughtherein.

As disclosed, ZIF circuit 108 produces common output signal 200 at afrequency sufficient for transmission. Thus, first frequency f₀ is equalto third frequency f₁ and may be transmitted as signal 102 afterappropriate amplification. Similarly, when received, signal 102 atfrequency f₁ may be passed directly via receiving module 116 to ZIFcircuit 108. Chain 110 i and 110 j may be configured substantiallysimilarly as one or more chains 110 e-110 g. However, the output signalfrom ZIF circuit 108 to be down-converted is at a frequency f₁, i.e.,the transmission frequency of chain 110 h; and the input signal to ZIFcircuit 108 is up-converted to frequency f₁. Accordingly, for theexemplary embodiment illustrated in FIG. 6, the exemplary MIMO system isa 3×3 system wherein the matrix of channel 104 is identical to Equations2 and 3. To prevent an unintended signal delay between chain 110 h andchains 110 i and 110 j, ZIF circuit 108 preferably output signal 200 tochain 100 d by an appropriate amount so that signal 102 is commonlytimed.

In accordance with one embodiment of the present invention, system 100 dmay be configured so that one of the local oscillator circuits is amaster oscillator circuit that permits all chains to output the samefrequency when a suitable switch places the master oscillator inoperative control of the other chain's filtering module. In accordancewith one embodiment of the present invention, chain 100 h may beeliminated and ZIF circuit 108 is in direct communication with a switch140.

FIG. 7 is a schematic view of a communication system in accordance withone or more further embodiments of the present invention, wherein thecommunications system comprises a plurality of linked radio frequencychains that, on a transmission side, apply the same frequency offset toa base frequency. System 100 f is preferably configured to be operativeusing MIMO architecture to efficiently transmit data between anothersystem 100 f and/or other compatible and/or suitably configured system.Thus, system 100 f may be operative with other systems 100 and comprisesessentially like architecture. Thus, the teachings of system 100, i.e.,100 a, 100 b, 100 c, 100 d and/or 100 e, are repeated here. However,system 100 f varies in certain aspects.

Advantageously, system 100 f provides a rugged architecture. System 100f permits the independent adjustments of the frequency of linked RFchains 110 with a linked multiple down-conversion or up-conversion.System 100 e comprises a plurality of chains 110. Illustrated in FIG. 7are four chains 110 k, 110 l, 110 m, and 110 n that are operative in anetwork 20 and configure a 2×2 MIMO system. However, any suitable numberof chains may be used. Each of the chains in system 100 e may beconfigured substantially similarly as chains 110 a-c. However, unlikethose chains, two or more chains in system 100 e are linked togetherinto a group 119 by utilizing a common oscillator circuit in the linkedchains. For example, the local oscillator circuit of chain 110 l may bemade inoperative or omitted and a local oscillator circuit 152 k ofchain 110 k may be made operative with chain 110 l to work in a linkedgroup 119 a. Similarly, chains 110 m and 110 n may be linked in a linkedgroup 119 b.

Thus, each group of linked chains generates a common transmittingfrequency. In the exemplary embodiment of FIG. 7, chains 110 k and 110 lgenerate a frequency f₁ and chains 110 m and 110 n generate a frequencyf₂. Thus, for the exemplary embodiment of FIG. 7, the matrix for channel104 is shown in Equation 4 or more explicitly in Equation 5, wherein asuperscript showing the transmitting frequency is indicated.

$\begin{matrix}{H = \begin{bmatrix}h_{11} & h_{12} & 0 & 0 \\h_{21} & h_{22} & 0 & 0 \\0 & 0 & h_{33} & h_{34} \\0 & 0 & h_{43} & h_{44}\end{bmatrix}} & \left( {{Equation}\mspace{14mu} 4} \right) \\{H = \begin{bmatrix}h_{11}^{f_{1}} & h_{12}^{f_{1}} & 0 & 0 \\h_{21}^{f_{1}} & h_{22}^{f_{1}} & 0 & 0 \\0 & 0 & h_{33}^{f_{2}} & h_{34}^{f_{2}} \\0 & 0 & h_{43}^{f_{2}} & h_{44}^{f_{2}}\end{bmatrix}} & \left( {{Equation}\mspace{14mu} 5} \right)\end{matrix}$Therein, the matrix coefficients are expressed as h_(TR), where T is thetransmitting module on a respective chain and R is the receiving moduleon a respective chain, namely “1” for chain 110 k, “2” for chain 110 l“3” for chain 110 m, and “4” for chain 110 n. It should be appreciatedthat if the number of chains that are present=n, the matrix may besuitably adjusted.

In accordance with one or more embodiments of the present invention, ineach linked group, the antenna associated with a chain that is a memberof the linked group may be advantageously adjusted for polarity. In theexemplary embodiment of FIG. 7, antenna 118 k and 118 l are associatedwith linked chains 110 k and 110 l, respectively. Therein, antenna 118 kand 118 l comprise a polarization that is orthogonal to the otherpolarization to advantageously improve signal propagation.

Antenna 118, e.g., antenna 118 k and/or 118 l, may comprise singleantenna with separate inputs for vertical and horizontal polarization,single antenna with separate inputs for dual slant diversity, singleantenna with separate inputs for circular polarization, and/or singleantenna with separate inputs for common polarization. Preferably, foreach linked chain, each transmitter side of the chain is operablyconnected to a dual input, polarization diversity antenna such that onechain corresponds to one polarization and a second chain corresponds toa second polarization.

At the receiver side, a similar antenna arrangement is made. Therein,for each linked chain, each receiver side of the chain is operablyconnected to a dual input, polarization diversity antenna such that onechain corresponds to one polarization and a second chain corresponds toa second polarization. Advantageously, the channel bandwidth is expandedby a factor of two, while reapplying known antenna polarizationtechniques to achieve an assured bandwidth typically double that of a2×2 MIMO system.

In accordance with one or more embodiments of the present invention,antennas 118 may be configured to be two separate antennas wherein eachcomprises inputs for vertical and horizontal polarization, dual slantdiversity, circular polarization and/or for a common polarization toallow beam steering of one frequency, e.g., frequency f₁, on the firstantenna and beam steering of another frequency, e.g., frequency f₂ onthe second antenna.

In accordance with one embodiment of the present invention, antenna 118may also be one common antenna comprising four inputs and a commonpolarization to allow beam steering of one frequency, e.g., frequencyf₁, on the first antenna and beam steering of another frequency, e.g.,frequency f₂ on the second antenna.

FIG. 8 is a schematic view of a communication system in accordance withone or more further embodiments of the present invention, wherein thecommunications system comprises a plurality of radio frequency chainsthat, on a transmission side, apply an independent frequency offset to abase frequency and utilize a combiner to combine a transmitting signal.System 100 g is preferably configured to be operative using MIMOarchitecture to efficiently transmit data between another system 100 gand/or other compatible and/or suitably configured system. Thus, system100 g may be operative with other systems 100 and comprises essentiallylike architecture. Thus, the teachings of system 100, i.e., 100 a, 100b, 100 c, 100 d and/or 100 e, are repeated here. Thus, system 100 may beoperative with other systems 100, such as systems 100 a-100 f. However,system 100 g varies in certain aspects.

Advantageously, system 100 g provides an architecture that limits thenumber physical antennas. System 100 g operatively permits theindependent adjustments of the frequency of RF chains 310 havingmultiple outputs and inputs. System 100 g comprises baseband mediaaccess controller 106, ZIF circuit 108, and a plurality of receivingand/or transmitting chains 310 operable with one or more receivingand/or transmitting antennas 118. Illustrated in FIG. 8 are two chains.However, any suitable number of chains may be used.

Each chain 310, i.e., chains 310 a and 310 b, preferably comprisesfiltering module 312 that includes a transmitter-side filteringsubmodule 311 and a receiver-side filtering submodule 313; a transmittercircuit 114 for transmitting signals 102 over channel 104 and/orreceiver circuit 116 for receiving signals 102 over a channel 104,depending on whether system 100 g is configured to respectively transmitonly, receive only, or both, and a switch 140 for switching betweentransmitting and receiving mode.

As taught herein, on a transmission side, at least one chain 310 isconfigured to down-convert a common output signal to a frequency that isdifferent than at least one other frequency in transmitting signals 102,e.g., apply a frequency offset to one transmission data stream. On areceiver side, the respective at least one chain 310 is configured toup-convert a frequency offset signal to a common frequency. Thereby, thebandwidth is effectively expanded and greater data delivery is assured.

Each transmitter-side filtering submodule 311 preferably comprises aplurality of initial mixer circuits 330, a plurality of filter circuits332, secondary mixer circuits 334 in communication with a respectivelocal oscillator circuit 352 x or 352 y, and a combiner 353. Each of themixer circuits 330 may be substantially identical to mixer circuit 130taught herein; filter circuits 332 may be substantially identical tofilter circuit 132, and oscillator circuit 352 may be substantiallyidentical to oscillator circuit 152; or each may be configured as anysuitable component of the type known in the art.

Each mixer circuit 330 is in communication with an output signal 200, astaught herein, of ZIF circuit 108 via physical layer 109. The outputsignal comprises a common first frequency f₀, wherein respectivereceivers and transmitter of system 100 are operable. Each mixer circuit330 of each chain preferably down-converts signal 200 at first frequencyf₀ received from the ZIF circuit 108 to an intermediate frequencyf_(IF), i.e., a second frequency f_(IF) to generate a second signal 202.Intermediate frequency f_(IF) may be different than any otherintermediate frequency in the same chain, submodule, and/or system. Eachinitial mixer circuit 330 is preferably in communication with arespective filter circuit 332, which may be substantially identical tofilter circuit 132, via an output. Filter circuits 332 filter thedown-converted signal 202 to a filtered down-converted signal 204 thatis received by a secondary mixer circuit 334.

System 100 g preferably includes one common oscillator circuit 351 thatis in operative communication with the plurality of initial mixercircuits 330 and a first and second local oscillator 352 x and 352 ythat are in communication with secondary mixer circuits 334. Eachoscillator may be configured as any other known oscillator, and, as oneskilled in the art will recognize by a plurality of oscillator linked toa common frequency source, i.e., clock. Each secondary mixer circuit 334is operatively connected to a different local oscillator circuit 352 xor 352 y and preferably is configured to up-convert the filtereddown-converted transmission signal 204 to a respective third frequency,i.e., transmission frequency, to generate a filtered transmission signal206. Filtered transmission signal 206 comprises transmission signal 200with the noise, distortion and other spurious signals removed orsubstantially reduced. Therein, preferably each transmission signalcomprises transmission frequency which differs from one or moretransmission frequencies in the same submodule in the exemplaryembodiment of FIG. 8, each chain includes frequencies f₁ and f₂.

Respective initial and secondary mixer circuits 330 and 334 provide adouble-conversion by translating the transmission signal 200 at firstfrequency f₀ to the lower, third frequency f₁ or f₂ for filtering, andthen translating the resulting filtered signal at intermediatefrequencies f_(IF) to higher, third frequencies for transmission.Respective transmission signals 206 are then combined in combiner 353into a filtered combined transmission signal 207. The combiner is incommunication with transmitter circuit 114, which is configured totransmit via a transmitter/receiver diversity switch 140 the filteredtransmission signal 207 to another system 100 in network 20 via antenna118. Combiner 353 may instead be provided after a bandpass filter hasfiltered transmission signal 206.

System 100 g may be configured to receive wireless signals via areceiving antenna, which can be the same or different antenna thanantenna 118. The signals received via the receiving antenna are passedvia the transmitter/receiver diversity switch 140 to a receiver circuit116. Receiver circuit 116 is configured to receive signals for thesystem 100 and may comprise a suitable bandpass filter 144, whichreceives and appropriately filters the received signals. The resultingbandpass-filtered signal is appropriately amplified by a suitablelow-noise amplifier 144 in communication with bandpass filter 142.Receiver circuit 116 and accompanying receiver components may includeadditional and/or alternative elements necessary for wireless or wiredsignal reception, depending on, for example, the type of signals beingreceived, the communication medium and protocol, and other like factors.The output of receiver circuit 116 is a received signal 209 at atransmitted frequency.

Each receiver-side filtering submodule 313 preferably comprises aplurality of initial mixer circuits 346 in communication with arespective local oscillator circuit 352 x or 352 y, a plurality offilter circuits 348, and a secondary mixer circuits 350, and a splitter355. Each of the mixer circuits 346 may be substantially identical tomixer circuit 146 taught herein; filter circuits 348 may besubstantially identical to filter circuit 148, and oscillator circuit352 may be substantially identical to oscillator circuit 152; or eachmay be configured as any suitable component of the type known in theart. Submodule 313 includes a splitter 355 that appropriately dividesreceived signal 209 into received signals 208 having a frequency f₁ anda frequency f₂. Each signal 208 is provided to initial receiver mixercircuit 346 which has an input in communication with an output of thesplitter. The splitter may be located any other place that is suitable.

Preferably, mixer circuits 346 are configured to receive received signal208 at transmitted frequency f₁. Third mixer circuit 346 may beconfigured to down-convert the received signal 208 at transmittedfrequency f₁ to an intermediate frequency f_(IF) to generate adown-converted received signal 210. Submodule 313 preferably includessecond filter circuits 348 in communication with an output of mixercircuits 346. Each second filter circuit 348 is configured to filter thedown-converted received signal 210 to generate a filtered down-convertedreceived signal 212 at the intermediate frequency f_(IF). Second filtercircuit 348 can comprise any suitable type of filter circuit or devicethat is capable of filtering noise, distortion and other spurioussignals from the down-converted received signal 210 at the intermediatefrequency f_(IF). Second filter circuit 348 may be configuredsubstantially similar to first filter circuit.

Submodule 313 preferably includes a mixer circuit 150 in communicationwith an output of the second filter circuit 148. Fourth mixer circuit350 is preferably configured to up-convert the filtered down-convertedreceived signal 212 to a base frequency f₀ to generate a filteredreceived signal 214. ZIF circuit 108 or other like transmitter,receiver, transceiver or communication circuit/device is incommunication with an output of the fourth mixer circuit 350 viaphysical layer input 109.

Filtered received signal 214 comprises received signal 208 with thenoise, distortion and other spurious signals removed or substantiallyreduced. The third and fourth mixer circuits 346 and 350 providesdouble-conversion of the received signal 208 at the transmittedfrequency f₁ to a lower intermediate frequency f_(IF) for filtering, andthen translating the resulting filtered signal to a higher, basefrequency f₀ for reception by the ZIF circuit 108 via physical layerinput. In this manner, the frequency offset is reversed to generate asignal for use by a controller that comprises the base frequency of thecommon output signal.

The exemplary embodiment of FIG. 8 comprises a 4×4 MIMO architecturewherein a frequency offset is created by using two local oscillators. Ineffect, system 100 g provides two 2×2 MIMO system, each connected to itsown antenna, wherein each system operates on two different frequenciesand have maximal ratio combining (“MRC”) gains. Advantageously, only twophysical antennas 118 are used. Thus, installation of a system 100 g maybe possible in locations having limited physical space, such a vehiclehaving limited roof space.

In accordance with one embodiment of the present invention, system 100 gcomprises a polarization diversity antenna such that one linked chaincorresponds to one polarization and a second linked chain corresponds toa second polarization. Therein, minimum cross polarization coupling XPDbetween the two RF channels minimizing adjacent channel emissions fromfrequency f₀ into frequency f₁ and vice versa, thereby allowing the twofrequencies f₀ and f₁ to be positioned closely together. At thereceiver, a similar antenna arrangement is made, with the linked chainsconnected to a polarization diversity antenna. It is required that thepolarization of corresponding pairs signals 102 are aligned.Advantageously, a bandwidth expansion by a factor of two is achieved,which is double that of two 1×1 single-input/single-output systems.Accordingly, channel 104 is identical to Equations 2 and 3 for theexemplary embodiment illustrated in FIG. 8.

FIG. 9 a is a schematic view of a communication system in accordancewith one or more further embodiments of the present invention, whereinthe communications system comprises a plurality of radio frequencychains that are configured to create a virtual antenna on a receiverside. System 100 h is preferably configured to be operative using MIMOarchitecture to efficiently transmit data between another system 100 hand/or other compatible and/or suitably configured system. Thus, system100 h may be operative with other systems 100 and comprises essentiallylike architecture. Thus, the teachings of system 100, i.e., 100 a-100 g,are repeated here. Thus, system 100 may be operative with other systems100, such as systems 100 a-100 g. However, system 100 h varies incertain aspects.

Advantageously, system 100 h is adapted for use with commerciallyavailable MIMO systems. Such systems limit the available physical layertransmission outputs from, for example, the ZIF circuit, but havegreater number of physical layer receiving inputs. System 100 hoperatively permits the independent adjustments of the frequency of RFchains 410 having a single physical layer output but multiple inputs. Inthis manner one receiving channel may be considered to be connected witha virtual antenna.

System 100 h comprises baseband media access controller 106, ZIF circuit108, and a plurality of receiving and/or transmitting chains 410operable with one or more receiving and/or transmitting antennas 118.Illustrated in FIG. 9 a are two chains. However, any suitable number ofchains may be used. Each chain 410, i.e., chains 410 a and 410 b,preferably comprises filtering module 412 comprising a transmitter-sidefiltering submodule 411 and a plurality of receiver-side filteringsubmodule 413; a transmitter circuit 114 for transmitting signals 102over channel 104 and/or receiver circuit 116 for receiving signals 102over a channel 104 depending on whether system 100 h is configured torespectively transmit only, receive only, or both, and a switch 140 forswitching between transmitting and receiving mode.

As taught herein, on a transmission side, at least one chain 410comprises a transmission-side filtering submodule 411. Submodule 411 isconfigured to receive a common output signal from a physical layeroutput, down-convert the common output signal, filter and amplify thesignal, and transmit it via a physical antenna. Furthermore, at least asecond chain 410 is configured to receive a common output signal from aphysical layer output, down-convert the common output signal to afrequency that is different than the frequency in another chain, i.e.apply a frequency offset, filter and amplify the signal, and transmitthe signal via a physical antenna.

On a receiver side, the respective at least one chain 410 comprises areceiver-side filtering submodule 413. Submodule 413 is configured toreceive a transmission signal from a physical antenna, filter andamplify it, split the filtered signal, and pass the signal to two ormore branches of the submodule. Each branch of submodule up-converts thesignal to a common frequency before passing the up-converted signals torespective physical layer inputs of the ZIF circuit. In this manner, twoor more up-converted signals may be obtained from a single physicalantenna of the MIMO system, e.g., the MIMO system comprises a physicalantenna and one or more virtual antennas.

Transmitter-side filtering submodule 411 preferably comprises a firstmixer circuit 430 in communication with a common oscillator circuit 451,a filter circuit 432, second mixer circuits 434 in communication with alocal oscillator circuit 452. First mixer circuit 430 may besubstantially identical to mixer circuit 130 taught herein; filtercircuit 432 may be substantially identical to filter circuit 132, andcommon oscillator 451 and local oscillator circuit 452 may besubstantially identical to local oscillator circuit 152; or each may beconfigured as any suitable component of the type known in the art. Firstmixer circuit 430 is in communication with an output signal 200 from aphysical layer output 409, as taught herein, of ZIF circuit 108comprising a common first frequency f₀, as taught herein, whereinrespective receivers and transmitters of system 100 are operable.

First mixer circuit 430 is in operative communication with commonoscillator circuit and preferably down-converts signal 200 at a commonfirst frequency f₀ from the ZIF circuit 108 to an intermediate frequencyf_(IF), i.e., a second frequency f_(IF) to generate a second signal 202.An output of first mixer circuit 430 is preferably in communication withfilter circuit 432 to pass the down-converted signal 202. Filter circuit432 filters signal 202 to filtered down-converted signal 204 that isreceived by second mixer circuit 434. Second mixer circuit 434 isoperatively connected to local oscillator circuit and preferablyup-converts the filtered down-converted transmission signal 204 to arespective third frequency, i.e., transmission frequency, to generate afiltered transmission signal 206.

Filtered transmission signal 206 comprises the transmission signal 200with the noise, distortion and other spurious signals removed orsubstantially reduced. Therein, preferably transmission signal 204comprises transmission frequency which differs from one or moretransmission frequencies of the other chains. In the exemplaryembodiment of FIG. 9 a, a first chain 410 a comprises a transmissionfrequency f₁ and while a second chain 410 b comprises a transmissionfrequency f₂.

Respective first and second mixer circuits 430 and 434 provide adouble-conversion by translating the transmission signal 200 at firstfrequency f₀ to respective intermediate frequencies f_(IF) to arespective third frequency f₁ or f₂ for filtering, and then translatingthe resulting filtered signal for transmission. Intermediate frequencyf_(IF) may vary between chains and submodules. In this manner, anadjustment, which is independent from adjustments by another chain, ismade to the base frequency, i.e., a frequency offset is applied, togenerate a signal for transmitting. The transmitted signal comprises afrequency that includes an offset from the frequency of the commonoutput signal wherein the difference in frequency between frequency f₀and frequency f₁ comprises the frequency offset.

Second mixer circuit 434 is in communication with transmitter circuit114, which is configured to transmit via a transmitter/receiverdiversity switch 140 the filtered transmission signal 206 to anothersystem 100 in network 20 via physical antenna 118. In the exemplaryembodiment of FIG. 9 a, ZIF circuit 108 comprises physical layer outputs409 a and 409 b, but other physical layer outputs are not.

System 100 h may be configured to receive wireless signals 102 via areceiving antenna, which can be the same or different antenna thanphysical antenna 118. The signals received via the receiving antenna arepassed via the transmitter/receiver diversity switch 140 to a receivercircuit 116. Receiver circuit 116 is configured to receive signals forthe system 100 and may comprise a suitable bandpass filter 144, whichreceives and appropriately filters the received signals. The resultingbandpass-filtered signal is appropriately amplified by a suitablelow-noise amplifier 144 in communication with bandpass filter 142.Receiver circuit 116 and accompanying receiver components may includeadditional and/or alternative elements necessary for wireless or wiredsignal reception, depending on, for example, the type of signals beingreceived, the communication medium and protocol, and other like factors.The output of receiver circuit 116 is a received signal 209.

Each receiver-side filtering submodule 413 preferably comprises aplurality of initial mixer circuits 446, a plurality of filter circuits448, and secondary mixer circuits 450 in communication with commonoscillator circuit 451 or any other suitable oscillator circuit, and asplitter 455. Each of the mixer circuits 446 may be substantiallyidentical to mixer circuit 146 taught herein; filter circuits 448 may besubstantially identical to filter circuit 148, or each may be configuredas any suitable component of the type known in the art.

Splitter 455, which may be located wherever suitable, preferably dividesfiltered received signal 209 into two or more received signals 208having a suitable frequency and is passed to a branch of the submodulebased on the orthogonality of the received signal 209. In the exemplaryembodiment of FIG. 9 a, each chain includes a receiver-side submodule413, which each has two or more branches, e.g., branches 413 x and 413y. Respective signal 208 is provided to two or more branches thatcomprise initial receiver mixer circuit 446, filter circuit 448 andmixer circuit 450. In each branch, mixer circuit 446 comprises an inputin communication with an output of the splitter.

Preferably, each initial mixer circuit 446 is preferably configured tobe in communication with a local oscillator circuit. In the exemplaryembodiment of FIG. 9 a, at least one mixer circuit 446 preferably is incommunication with local oscillator 452 x, which also in communicationwith mixer circuit 434, while one or more other mixer circuits are incommunication with local oscillator circuit 452. Each mixer circuit 446is configured to receive signal 208 at frequency f₁ and down-convert thereceived signal 208 from frequency f₁ to an intermediate frequencyf_(IF) to generate a down-converted received signal 210. Intermediatefrequency f_(IF) may vary between branches.

Filter circuit 448 is in communication with an output of mixer circuit346 and filters the down-converted received signal 210 to generate afiltered down-converted received signal 212 at the intermediatefrequency f_(IF). Filter circuit 348 can comprise any suitable type offilter circuit or device that is capable of filtering noise, distortionand other spurious signals from the down-converted received signal 210at the intermediate frequency f_(IF). Secondary mixer circuit 450 is incommunication with an output of filter circuit 448 and up-converts thefiltered down-converted received signal 212 to a frequency f₀ togenerate a filtered received signal 214 that is provided to ZIF circuit108 via physical layer inputs 109 b. Filtered received signal 214comprises received signal 208 with the noise, distortion and otherspurious signals removed or substantially reduced.

In the exemplary embodiment of FIG. 9 a, the initial and secondary mixercircuits 446 and 450 provide provides double-conversion of the filteredreceived signal 209 from a frequency f₁ to a frequency f₀ for input intoZIF circuit 108 via physical layer inputs 109 by dividing the filteredreceived signal 209 into one or more signals 208 that are passed tobranches of the submodule. In this manner, the frequency offset isreversed to generate a signal for use by a controller that comprises afrequency of the common output signal.

The exemplary embodiment of FIG. 9 a comprises a 2×4 MIMO architecturewherein a frequency offset is created by using two local oscillatorcircuits. In effect, system 100 h provides two 2×2 MIMO system, eachconnected to its own antenna, wherein each system operates on twodifferent frequencies and have maximal ratio combining (“MRC”) gains.Advantageously, only two physical antennas are present. By dividing thefiltered received signal, one or more branches may be configured to havea virtual antenna 118 x. In this manner, fewer physical antennas areneed permitting installation in applications having limited physicalspace. Also advantageously, even though a limited number of transmittersare used, a greater number of receivers are used.

Thus, for the exemplary embodiment of FIG. 9 a, the matrix for channel104 is shown in Equation 6 or more explicitly in Equation 7, wherein asuperscript showing the transmitting frequency is indicated.

$\begin{matrix}{H = \begin{bmatrix}h_{11} & 0 \\0 & h_{22} \\h_{31} & 0 \\0 & h_{42}\end{bmatrix}} & \left( {{Equation}\mspace{14mu} 6} \right) \\{H = \begin{bmatrix}h_{11}^{f_{1}} & 0 \\0 & h_{22}^{f_{2}} \\h_{31}^{f_{1}} & 0 \\0 & h_{42}^{f_{2}}\end{bmatrix}} & \left( {{Equation}\mspace{14mu} 7} \right)\end{matrix}$

FIG. 9 b is a schematic view of an embodiment of a communications systemof FIG. 9 a. Therein, a plurality of common oscillator circuits areprovided and operatively connected. Thus, a system 100 i rather thancomprising local oscillator circuits as in system 100 h, commonoscillator circuit 451 is operative with all mixer circuits of thetransmission-side filtration submodule and one branch of thereceiver-side filtration submodule. Common oscillator circuit 451 is infurther communication with one or more branches of a receiving sidesubmodule of other chains 410. Similarly, one or more other commonoscillator circuits 451 of another chain is in communication with arespective branch of the receiver-side filtration submodule such thateach branch of the receiver-side filtration submodule is incommunication with a different common oscillator circuit. Thus, for theexemplary embodiment of FIG. 9 b, the matrix for channel 104 is shown inEquation 6 or more explicitly in Equation 7, wherein a superscriptshowing the transmitting frequency is indicated.

FIG. 9 c is a schematic view of an embodiment of a communications systemof FIG. 9 a. Therein, system 100 j comprises a receiver-side submodule413 using one or more selectable frequency gain circuit 413 z forenabling MRC gains. The circuit is in communication with a physicallayer input 109 to transmit the resulting signal for advantageouslybeing used with FM radio reception and detection. If the gain circuitsare not selected, additional MRC gains will not be realized. Thus, forthe exemplary embodiment of FIG. 9 c, the matrix for channel 104 isshown in Equation 8 or more explicitly in Equation 9, wherein asuperscript showing the transmitting frequency is indicated.

$\begin{matrix}{H = \begin{bmatrix}h_{11} & 0 \\0 & h_{22} \\h_{31} & h_{32} \\h_{31} & {- h_{32}}\end{bmatrix}} & \left( {{Equation}\mspace{14mu} 8} \right) \\{H = \begin{bmatrix}h_{11}^{f_{1}} & 0 \\0 & h_{22}^{f_{2}} \\h_{31}^{f_{1}} & h_{32}^{f_{2}} \\h_{31}^{f_{1}} & {- h_{32}^{f_{2}}}\end{bmatrix}} & \left( {{Equation}\mspace{14mu} 9} \right)\end{matrix}$

FIG. 9 d is a schematic view of a communication system in accordancewith one or more further embodiments of the present invention whereinthe communications system comprises a plurality of radio frequencychains that are configured to create a virtual antenna on a receiverside. System 100 k is preferably configured to be operative using MIMOarchitecture to efficiently transmit data between another system 100 kand/or other compatible and/or suitably configured system. Thus, system100 i may be operative with other systems 100 and comprises essentiallylike architecture. Thus, the teachings of system 100, i.e., 100 a-100 h,are repeated here. Thus, system 100 may be operative with other systems100, such as systems 100 a-100 h.

System 100 i is substantially similar to system 100 h. However, system100 i provides for additional combining of the filtered received signal214 by combining a signal of one branch 413 x of one receiver-sidesubmodule 413 with another branch 413 x of one receiver-side submodule413 using one or more selectable frequency gain circuit 413 z forenabling MRC gains. The circuit is in communication with a physicallayer input 109 to transmit the resulting signal. If the gain circuit isnot selected, additional MRC gains will not be realized.

The exemplary embodiment of FIG. 9 d comprises a 2×3 MIMO architecturewherein a frequency offset is created by using two local oscillatorcircuits. In effect, system 100 i is connected only two physicalantennas, but has a greater number of receiver-side physical inputs thanantennas. By dividing the filtered received signal, one or more branchesmay be configured to have a virtual antenna 118 x. In this manner, fewerphysical antennas are need permitting installation in applicationshaving limited physical space. Also advantageously, even though alimited number of transmitters are used, a greater number of receiversare used.

In accordance with one or more embodiments of the present invention,system 100 may comprise baseband controller wherein signal outputs 200comprise different frequencies rather than a common frequency f₀.

In accordance with one or more embodiments of the present invention,system 100 comprises RF converter in communication with a basebandcontroller. The RF converter preferably receives MIMO baseband inputsand then converts them to independently tunable RF outputs.

In accordance with one or more embodiments of the present invention,system 100 is independent of MIMO technology, whether it be 802.11n,802.16d, 802.16e, or other possible future wireless technologies whichemploy multiple-input/multiple-output (MIMO) technology for both samefrequency transmissions as well as frequency shifting, i.e., frequencyoffsetting, transmissions.

In accordance with one or more embodiments of the present invention,system 100 may used for multimode or restricted multimode optical fibersystems as a means of improving throughput.

FIG. 10 a is a schematic view of a ZIF circuit in accordance with one ormore embodiments of the present invention. A ZIF circuit 508, such asZIF circuit 108, may be configured to produce independently tunable ZIFoutput frequencies to supply a communications system, such ascommunications system 100. For clarity, the communications system isreferred to as communications system 500 and comprise any suitablecommunications system, especially any embodiment of communicationssystem 100, e.g., 100 a-100 h. ZIF circuit 508 may be operative with abaseband media access controller, such as baseband controller 106. ZIFcircuit 508 is preferably configured to comprise a plurality ofindependently tunable RF chains 510 that is operative with an antenna,such as antenna 118, to generate a plurality of signals 102 comprising aplurality of frequencies f₁, f₂, . . . f_(N) for transmission andreception of signals comprising matched frequencies f₁, f₂, . . . f_(N).

FIG. 10 b is a schematic view of the ZIF circuit of FIG. 10 a inaccordance with one or more embodiments of the present invention.Therein, communications system 500 comprises three RF chains 510 and afrequency synthesizer 501 that are integrated in ZIF circuit 508 toproduce a plurality of frequencies. Preferably, frequency synthesizer501 is a common synthesizer used in all RF chains 510 a of ZIF circuit508.

FIG. 10 c is a schematic view of a detail of an RF chain of FIG. 10 b inaccordance with one or more embodiments of the present invention. RFchain 510 is configured as RF chain 510 a comprising substantially RFchain 110 i integrated on the ZIF circuit for, for example, a 3×3 MIMOsystem. RF chain 510 a comprises in-phase and quadrature-phase (IQ)input signals 600 and a frequency synthesizer 501, which may be anysuitable synthesizer, to apply a single frequency offset in order to mixI/Q signals 600 to an RF output signal 606 at a predetermined frequency,such frequency f₂, . . . f_(N). The frequency synthesizer is connectedto a crystal 625 that provides a frequency reference.

Signal 600 at a baseband frequency, such as frequency f₀, of the Ibranch is passed to a low pass filter circuit 502 a in communicationwith a mixing circuit 504 a, while signal 600 of the Q branch issimilarly passed to a low pass filter circuit 502 b in communicationwith a mixing circuit 504 b. Each of the mixer circuits is incommunication with the synthesizer 501, which preferably comprises RFoscillator circuit or the like including a suitable Phase Locked Loop(“PLL”) oscillator circuit or the like. The mixer circuits apply afrequency offset to signal 600 to achieve a signal 606 at apredetermined frequency, such frequency f₁, f₂, . . . f_(N), whereineach chain of ZIF circuit 508 comprises a different one of thesefrequencies.

Respective RF output signal 606 of the I and Q branches is joined andthen passed to a transmitter circuit 514 comprising a bandpass filterand an amplifier. In turn, the transmitter circuit passes signal 606 toa switch 540 that is operative with an antenna 118. A signal 102received from antenna 118 is passed via switch 540 to a receivingcircuit 516 comprising a bandpass filter and an amplifier. Mixercircuits 504 c and 504 d are operative with the amplifier to receivedsignal 608 from the amplifier. Signal 608 comprises one of thepredetermined frequency, such frequency f₁, f₂, . . . f_(N). Mixercircuits 504 c and 504 d are operative with synthesizer 501 toup-convert signal 608 to a signal 614 comprising the baseband frequency,such as frequency f₀ for the I and Q branches. A pair of low pass filter548 a, 548 b are operatively connected to amplifiers and pass signal 614to each of the I and Q branches.

Advantageously, RF chain 510 a comprises a “true” ZIF, since there is nointermediate frequency produced. Herein, chain 510 a utilizes a singlefrequency synthesizer frequency offset to mix I/Q signals up to RFoutputs. Preferably, the synthesizer is configured to the desired RFoutput frequency, for example 5.4 GHz. Also, preferably, synthesizer 501comprises a common synthesizer used in each of the RF chains 510 a ofZIF circuit 508.

FIG. 10 d is a schematic view of a detail of an RF chain of FIG. 10 b inaccordance with one or more further embodiments of the presentinvention. RF chain 510 is configured as RF chain 510 b comprisingsubstantially multiple integrated on the ZIF circuit for, for example, a3×3 MIMO system. On the transmission side, RF chain 510 b is configuredsubstantially similarly as RF chain 510 a. However, chain 510 b differsin some aspects. RF chain 510 b comprises a first mixer 504 a, 504 b anda second mixer 506 a to internally generate an intermediate frequency,and then up convert that frequency to an RF output signal 606. On thereceiving side, RF chain 510 b is configured substantially similarly asRF chain 510 a. However, chain 510 b differs in some aspects. RF chain510 b comprises a second mixer 506 b to internally generate anintermediate frequency from received signal 608 and a first mixer 504 c,504 d that up convert that frequency to baseband input signal 614.Herein, synthesizer 501 is configured as a fractional synthesizer 501 a,which may be a ⅓-⅔ synthesizer (as shown), where the synthesizer isoperating at 3.6 GHz, and provides both 1.8 GHz and 3.6 GHz outputs tomix the RF signal from IQ baseband up to 3.6 GHz (i.e., the IFfrequency) and then up to 5.4 GHz.

FIG. 10 e is a schematic view of the ZIF circuit of FIG. 10 a inaccordance with one or more embodiments of the present invention.Therein, communications system 500 comprises three RF chains 510 eachcomprising an independent frequency synthesizer 501 b, wherein eachchain is integrated in ZIF circuit 508 to produce a plurality offrequencies. Preferably, frequency synthesizer 501 is a commonsynthesizer used in all RF chains 510 a of ZIF circuit 508. Despite thefact that RF chain generates an intermediate frequency, the intermediatefrequency is not detected and rather the outputs appears to be a ZIFoutput, since the internal IF is not seen externally to the ZIF circuit.

FIG. 10 f is a schematic view of a detail of an RF chain of FIG. 10 e inaccordance with one or more embodiments of the present invention. RFchain 510 is configured as RF chain 510 c comprising substantiallymultiple integrated on the ZIF circuit for, for example, a 3×3 MIMOsystem. RF chain 510 c is configured substantially similarly as RF chain510 a. However, chain 510 c differs in some aspects. RF chain 510 cincludes a single mixer circuit for each of the I and Q branches.However, each chain 510 c comprises a independent synthesizer 501configured as an independent frequency synthesizer 501 b to apply afrequency offset to the I/Q signals 600 to achieve a signal 606 at apredetermined frequency, such frequency f₁, f₂, . . . f_(N), whereineach chain of ZIF circuit 508 comprises a different one of thesefrequencies.

FIG. 10 g is a schematic view of a detail of an RF chain of FIG. 10 e inaccordance with one or more further embodiments of the presentinvention. RF chain 510 is configured as RF chain 510 d comprisingsubstantially multiple integrated on the ZIF circuit for, for example, a3×3 MIMO system. RF chain 510 d is configured substantially similarly asRF chain 510 b. However, chain 510 d differs in some aspects. RF chain510 d comprises a first mixer 504 and a second mixer 506 to internallygenerate an intermediate frequency, and then up convert that frequencyto an RF output signal 606. However, each chain 510 d comprises anindependent frequency synthesizer 501 a to apply a frequency offset tothe I/Q signals 600 to achieve a signal 606 at a predeterminedfrequency, such frequency f₁, f₂, . . . f_(N), wherein each chain of ZIFcircuit 508 comprises a different one of these frequencies. Therein, RFchains 510 c and 510 d utilize preferably different synthesizers 501 afor all radio Tx/Rx chains in the same ZIF circuit 508 to apply afrequency offset to the IQ signals to generate signals at an IFfrequency, and then to different RF frequencies.

FIG. 10 h is a schematic view of a detail of an RF chain of FIG. 10 e inaccordance with one or more further embodiments of the presentinvention. RF chain 510 is configured as RF chain 510 e comprisingsubstantially multiple integrated on the ZIF circuit for, for example, a3×3 MIMO system. RF chain 510 e is configured substantially similarly asRF chain 510 c. However, chain 510 e differs in some aspects. In lieu ofa crystal, RF chain 510 e includes a voltage-controlled oscillator (VCO)630 and a digital-to-analog converter (DAC) 635 for providing afrequency reference to the frequency synthesizer 501 b. It is noted thatthe VCO 630 and the DAC 635 may be integrated into a single component.Additionally, RF chain 510 e includes a baseband processor 640, whichaccepts all of the I and Q branches as inputs. The baseband processor640 extracts a carrier frequency offset (CFO) from the I and Q inputs,and then uses an algorithm according to an embodiment of the presentinvention to control the DAC 635 and the VCO 630.

In accordance with one or more embodiments of the present invention,switch elements may be provided between each synthesizer 501 b to permitone or more synthesizers to control two or more chains 510 for operatingindependent chains 510 to operate in standard MIMO mode on the samefrequencies. It should be appreciated that the ZIF circuit 508 may beconfigured in many different ways by, for example, utilizing differentsynthesizer designs, different filtering, the use of differentialsignals and summing at RF rather than IF, and these are contemplated inthe present invention.

FIG. 11 is a perspective view of a portion of a communications networkin accordance with one or more embodiments of the present invention. Aportion of network 20 comprises physical points of presence networkelement 22 disposed operably on a physical structure 24, such as a lightpole. Each point of presence network element comprises any suitablecommunications system 100 taught herein and is preferably provided in asuitable weather-tight operative physical embodiment as is known in theart.

Each communication system is preferably in operative communication withanother communications system 100 using MIMO technology with a pluralityof frequencies, wherein at least one frequency comprises an offsetfrequency, to be functional as a network backhaul. For example, onecommunications system at a first network element 22 a may use signals102 comprising a first and second frequency f₁ and f₂ to communicatewith another communications system at a second network element 22 b. Astaught herein, at least one of the frequencies f₁ or f₂ comprises afrequency offset to permit efficient communications using MIMOtechnology. Second network element 22 b is communication not only withthe communications system of the first network element, but also acommunications system 100 of a third network element 22 c using signals102 comprising different frequencies f₃ and f₄, wherein one or morefrequencies comprises a frequency offset to permit efficientcommunications using MIMO technology. Thus, by passing signals 102 fromone network element to a subsequent network element, signals 102 mayreach a junction with a landline or backbone.

In accordance with one or more embodiments of the present invention, acost effective backhaul means may be provided. A network element 22comprising any system 100 may be in communication by sending differentfrequencies of the MIMO channel to different points of presence usingdifferent physical or virtual antennas. For example, network element 22a may be in communication with network element 22 b via frequency f₁ andin communication with network element 22 c via frequency f₂ by suitablyorienting the antenna nodes.

The channel matrix for system 100 to network element 22 b may compriseonly a single coefficient h₁₁ for frequency f₁ and matrix coefficienth₂₂ approaches zero while the matrix of channel 104 for system 100 tonetwork element 22 b may comprise only a single coefficient h11approaches zero and while matrix coefficient h22 for frequency f₂ isnon-zero. Therein, system 100 comprises a low cost radio capablesupporting frequency specific radio links to different systems using asingle-input/single-output system.

FIG. 12 is a perspective view of the portion of the communicationsnetwork of FIG. 11 wherein user communication devices are operative withthe network. Advantageously, the MIMO architecture of system 100incorporating a frequency offset improves the bandwidth is effectivelyexpanded and greater data delivery is assured for mobile or staticdevice users. Therein, user 30 a in the form of a vehicle utilizing avehicle-borne system 100 is configured as a mobile device user, whileuser 30 b utilizing a cellular telephone based system 100 is configuredas a static device user. Certain vehicles, such as police cruisers, lacksufficient space, especially on the roof, for all communication devices.Thus, these users are permitted only a single antenna for data services.Advantageously, by utilizing a system 100, specifically system 100 h or100 i, additional physical antennas are not needed because the systemutilizes virtual antennas.

In accordance with one or more embodiments of the present invention, auser interface may be provided to control system 100 via a controlsystem. The interface and/or control system may be configured as acommand line interface, a graphical user interface, web-based graphicaluser interface, or a linked networked management system. The controlsystem sets and monitors the status of the frequencies associated witheach channel 104. Therein, the control system is able to select one ormore frequencies in one or more systems 100 based on known or detectedinterference to minimize the interference. For example, the controlsystem may configure operative systems 100 such that a first system 100is in communication with a second system 100 using a channel in UNII2,while third and fourth systems 100 in possible interfering vicinity areoperative in a channel in an ISM band.

Even therein, vastly different ISM bands such as 928 MHz and 5.47 GHzare available to be selected by the control system. For example, onechannel may be operative at 928 MHz, which selected for its increasedreach and lower attenuation through foliage, or a channel in a licensedband (2.5-2.7 GHz and a second channel in an unlicensed band may beselected to allow for guaranteed services in the licensed band andadditional bandwidth in the unlicensed band.

In accordance with one or more embodiments of the present invention,system 100 comprises an antenna diversity switch and/or control systemfor one or more of the receiver-side transmission chains to select thestrongest channel. For example, system 100 may be configured to permitbeam steering to one or more antennas 118 whether the chains 100 arelinked or not. Thus, antenna 118 or any other suitable antenna maycomprise parallel radiating elements to enable beam steering.

The present invention describes a method and an apparatus forsynchronizing the receivers to the transmitters, or vice versa, for aMIMO-based frequency shifted system. In a preferred embodiment, themethod of the present invention is used in a point-to-point (P2P) orpoint-to-multipoint (P2MP) backhaul system where a master-slave orsimilar (e.g., AP-Client or BS-MS) arrangement exists. A control systemis run on the slave system. The control system employs the carrierfrequency offset of the received packets from the master system (or viceversa, or distributed so that algorithms are run on both master andslave) to adjust and control the clock recovery of the slave system,thereby aligning it to the master system.

Wi-Fi and WiMAX chips use the pilot tones from an incoming packet toestimate the carrier frequency offset (CFO) as it processes the OFDMsignals. These chips often estimate the course frequency error duringthe synchronization symbols, and may employ a Coordinate RotationalDigital Computer (CORDIC) functional element to determine a moreaccurate CFO that is calculated while a packet is being received.Normally, the CFO is used for debugging purposes, and is present in thelow level physical interface.

The CFO represents the difference in the carrier frequency of atransmitted packet with respect to the receiver's clock frequency. Forexample, if a packet is transmitted by a master system at 6 GHz+10 pp,then the actual transmission frequency will be 6,000,060,000 Hz. If thatsame packet is received by a Wi-Fi or WiMAX radio with a crystal at −10ppm (i.e., 5,999,940,000 Hz), then the CFO will be 6,000,060,000Hz−5,999,940,000 Hz=120,000 Hz=120 kHz.

The present invention uses the CFO to offset the main oscillator on theslave device to align it to the master device. By adjusting the slavecrystal to be +10 ppm, the CFO will read a value of 0 Hz, indicatingthat the two clocks are locked. The reason for locking the two clocks isto enable the MIMO bandwith expansion techniques to be employed withoutrequiring expensive crystals, or GPS, or IEEE 1588-based timing circuitsor alternate frequency locking methods to be employed.

In practice, the crystals will not be exactly locked. A small error inclock frequencies will increase the error vector magnitude (EVM) of thedemodulated signal. However, the EVM and the magnitude of the clockfrequency error must be kept within an acceptable tolerance range. Twomain factors determine the level of an acceptable clock recovery. Thefirst is how far apart the MIMO signals are transmitted, and the secondis the maximum allowed EVM contribution due to clock error.

Referring to FIG. 13, the present invention enables low cost adjustableoscillators to be used for MIMO frequency expansion techniques. Althoughit can be used for any carrier frequency into the tens or hundreds ofgigahertz, in a preferred embodiment, the carrier frequency is below 6GHz, as these frequencies are particularly applicable to RF linksdeployed on light poles, wooden poles, or just above street level. Below6 GHz, the available bands include 4.94-4.99 GHz; 5.15-5.85 GHz;5.85-5.925 GHz; and 2.4-2.4825 GHz for Wi-Fi; or 2.3-2.36 GHz, 2.5-2.7GHz, and 3.5-3.785 GHz for WiMAX-type transmissions. In general, theworst case expected condition exists when the MIMO expansion movesfrequencies apart by approximately 1 GHz, such as is the case where onesignal is in the 5.8 GHz ISM band and a second signal is in the 4.94 GHzpublic safety band, or a WiMax signal is transmitted at 2.5 GHz and asecond signal is transmitted at 3.5 GHz. Therefore, for these reasonsand for simplicity of calculations, it is assumed that a reasonablefrequency separation of the MIMO signals is equal to 1 GHz.

For a 64 quadrature amplitude modulation (64 QAM) signal, the EVM mustbe less than or equal to −24 decibels. In a preferred embodiment of theinvention, the error vector magnitude (EVM) is less than or equal to −30dB, thus providing a 6 dB margin. A 3 dB increase in EVM to −27 dB wouldbe acceptable and not excessively onerous. For an EVM less than or equalto −27 DB, the error introduced by the clock error must be less than orequal to −30 dB. This is an acceptable EVM contribution for Wi-Fi 64 QAMsignals. The maximum EVM contribution may need to be lower for 256 QAMor 1024 QAM signals. Accordingly, a goal of the present invention is toallow signals to be separated by up to 1 GHz and to have the carrierfrequency offset be small enough to have less than a 3 dB reduction inEVM for a 64 QAM signal.

EVM is a measurement of modulator performance in the presence ofimpairments. The soft symbol decisions obtained after decimating therecovered waveform at the demodulator output are compared against theideal symbol locations. The root mean square (rms) EVM and phase errorare then used in determining the EVM measurement over a window of Ndemodulated symbols.

Referring to FIG. 11, the symbol decision output by the demodulator isgiven by w and the ideal symbol location (using the symbol map) is givenby v. The resulting error vector is the difference between the actualmeasured and ideal symbol vectors: e=w−v. The error vector e isgraphically represented in FIG. 11: v is the ideal symbol vector; w isthe measured symbol vector; w−v is the magnitude error; θ is the phaseerror; e=w−v is the error vector, and e/v is the EVM.

For a normalized symbol vector |v| of unity, and given that the errorvector e is largely caused by rotation and thus is approximatelyperpendicular to v, then for small errors, e/v=sin(θ)≈θ. The acceptableEVM is 20*log(e/v)=−30 dB. Therefore e/v≈θ=10-30/20, and v=1, thusθ=0.032 radians=2 degrees. For a 2 degree error during a 250 μs packet,the allowed carrier frequency error is (2/360/0.25 ms)=22 Hz. For a 22Hz error, under the worst case condition of a 1 GHz frequency offset,the clocks must be locked to 22/1 GHz=22 parts per billion (ppb).Although this may appear to be an aggressively tight specification,because it represents a clock error which is approximately 1000 timesbetter than most off-the-shelf crystals which are specified to ±20 ppm,it is in fact easily achievable with a real-time control algorithm givena real-time stream of ten or more CFO estimates on the order of 2000 Hzor better per second. Using simple techniques, such as least meansquared error or Kalman filter algorithms, noise from CFO measurementsderived on a packet-by-packet basis can be extracted and minimized tothe point where the clock frequencies are locked to within 66 Hz.

As an example, BelAir Networks has developed similar timing capabilitiesfor circuit emulation services as described in U.S. patent applicationSer. No. 11/963,524, entitled “Method for Estimating and MonitoringTiming Errors in Packet Data Networks”, the contents of which areincorporated herein by reference in their entirety. Referring to FIG.12, exemplary data are shown which indicated clock recovery errors onthe order of less than 10 ppb. As a result, the required clock accuracychanges from 22 ppb to 550 ppb, which can easily be achieved using lowcost off-the-shelf voltage controlled oscillators (VCOs) withouttemperature compensation. Accordingly, this embodiment relies upon asimple VCO system, with the ability to extract CFO measurements from thereceived packet data stream.

In another embodiment of the present invention, the system may notrequire a tolerance of 22 ppb. By comparison with the embodimentdescribed above, the only parameter that is changed is the allowedfrequency offset. By placing frequency-shifted channels as contiguous 40MHz blocks, the frequency tolerance is thereby reduced from 1 GHz to 40MHz, i.e., a factor of 25.

In another embodiment, alternative means for locking the physical layersmay be employed. Two of the most practical such means include globalpositioning system (GPS) timing sources and IEEE 1588 timing referencesources. GPS clocks are widely used today to lock WiMAX data streams forthe purposes of ensuring timing and frequency accuracy; and aligningbase transceiver station (BTS) timing sources to allow multi-radio BTSunits to align all of their transmitters. Such an alignment ensures thatthe high power radios are not transmitting while they are receiving,which would otherwise result in significant degradations in receiversensitivities, because common antennas are used for multiple radios.Although the BTS systems are timed via a GPS source, conventional BTSsystems have not been known to employ this timing for the purpose ofMIMO frequency shifting prior to the present invention.

The problem with GPS timing is the inability to rely on these solutionsin urban canyons, where surrounding tall buildings effectively cut offline-of-sight paths to satellites required for timing generation. Thisis not an issue for roof-mounted GPS systems, but it is an issue forstreet-level-mounted GPS timing systems.

The second alternative means is the use of an IEEE 1588 timing source,where all wireless units employ an IEEE 1588 timing receiver block whichcommunicates to one or more timing servers using IP packet data totransfer timing information. These systems are not limited by the same“urban canyon” issues as GPS timing systems; however, IEEE 1588 systemshave not been known to be reliably employed to generate GPS stratumlevel timing.

Finally, other means may be employed, such as oven-controlled,temperature-compensated crystal oscillators (OCTCXOs). Such devices areavailable with an accuracy of 100 ppb, and it is expected that thesesources may improve to achieve 50 ppb with new technologies that monitorand address aging. However, the use of an OCTCXO is relatively expensiveby comparison with other means described above.

It will be appreciated by those of ordinary skill in the art that thepresent invention can be embodied in various specific forms withoutdeparting from the spirit or essential characteristics thereof. Thepresently disclosed embodiments are considered in all respects to beillustrative and not restrictive. The scope of the invention isindicated by the appended claims, rather than the foregoing description,and all changes that come within the meaning and range of equivalencethereof are intended to be embraced.

What is claimed is:
 1. A method of synchronizing a receiver with atransmitter in a communications system comprising amultiple-input/multiple-output (MIMO) architecture, the architecturecomprising a first and a second radio frequency chain, the receiverbeing connected to a controller, the communications system beingconfigured to transceive at least two signals having a predeterminedfrequency separation, and the method comprising the step of locking afrequency of the receiver to a frequency of the transmitter byconfiguring the controller to perform the steps of: a) using a packettransmitted by the transmitter and received by the receiver to determinean error associated with the transmitted packet; b) adjusting thereceiver reference frequency based on the determined error; and c)repeating steps a) and b) until the determined error is substantiallyequal to zero.
 2. A method of synchronizing a receiver with atransmitter in a communications system comprising amultiple-input/multiple-output (MIMO) architecture, the architecturecomprising a first and a second radio frequency chain, the receiverbeing connected to a controller, the communications system beingconfigured to transceive at least two signals having a predeterminedfrequency separation, and the method comprising the step of locking afrequency of the receiver to a frequency of the transmitter byconfiguring the controller to perform the steps of: a) using a packettransmitted by the transmitter and received by the receiver to determinean error associated with the transmitted packet; b) adjusting thereceiver reference frequency based on the determined error; c) using aretransmission of the received packet to determine an updated errorassociated with the retransmitted packet; and d) repeating steps b) andc) until the determined updated error is substantially equal to zero.